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The neurofibromatosis 2 (NF2) tumor suppressor gene encodes multiple alternatively spliced transcripts.

Neurofibromatosis type 2 (NF2) is an autosomal dominantly-inherited disorder predisposing affected individuals to tumors of multiple cell types in the central nervous system, including meningiomas. A candidate tumor suppressor gene for this disorder has recently been cloned; the protein product of this gene has a predicted role in linking integral membrane proteins with the cytoskeleton. Utilizing reverse transcription-polymerase chain reaction (RT-PCR) analyses, we have identified a number of alternatively spliced transcription products encoded by the NF2 gene. These alternative splice variants were detected in RNA isolated from several sources, including primary leptomeningeal tissue and an established line of leptomeningeal cells (LMC). Several of these variants delete previously identified coding regions of this gene. Moreover, two of these splice variants add previously unrecognized exons to the NF2 coding region. These identified splice forms will serve as natural reagents for the functional dissection of the NF2 protein product(s). They also should be considered in studies investigating mutations of this gene in members of NF2 families and in tumor analyses.

Amino Acid Sequence↗

Polymorphisms/mutations of TCR-zeta-chain promoter and 3' untranslated region and selective expression of TCR zeta-chain with an alternatively spliced 3' untranslated region in patients with systemic lupus erythematosus.

A vast majority of systemic lupus erythematosus (SLE) patients display decreased expression of TCR zeta-chain mRNA, a critical signaling molecule implicated in the selection of the TCR repertoire and in the prevention of autoimmunity. To identify the molecular mechanisms involved in the downregulation of TCR zeta-chain transcripts in SLE T cells, we investigated the possibility of polymorphisms/mutations in the promoter and the 3' untranslated region. PCR, cloning and sequence analysis of the promoter region from the genomic DNA showed significantly higher number of polymorphisms in SLE T cells compared to non-SLE control subjects (P = 0.044). Promoter sequence was also analysed from granulocytes to delineate the possibility of somatic mutations in activated SLE T cells. Promoter polymorphisms were significantly higher in granulocytes of SLE patients compared to non-SLE controls (P = 0.048), suggesting that these polymorphisms were of genomic origin. Nucleotide analysis of the promoter sequence revealed a -76T insertion compared to the published sequence, in all of the SLE samples and controls. RT-PCR analysis of the TCR zeta-chain 3' untranslated region showed a 344 bp product in addition to the expected 906 bp product. Cloning and sequence analysis of the 344 bp product indicated that it is an alternatively spliced form with both splicing donor and acceptor sites, resulting in deletion of nucleotides 672-1233 of TCR zeta-chain mRNA. Unlike the nomal TCR zeta-chain, the expression of TCR zeta-chain with the alternatively spliced 344 bp 3' untranslated region was higher in SLE T cells compared to non-SLE controls. The number of mutations/polymorphisms in the 906 bp TCR zeta-chain 3' untranslated region were significantly higher in SLE T cells compared to non-SLE subjects (P = 0.032). Frequent mutations/polymorphisms and aberrant splicing of the downstream 3' untranslated region may affect the stability and/or transport of TCR zeta-chain mRNA, leading to its downregulation in SLE T cells.

3' Untranslated Regions↗

Characterization of alternatively spliced products and tissue-specific isoforms of USP28 and USP25.

BACKGROUND: The ubiquitin-dependent protein degradation pathway is essential for the proteolysis of intracellular proteins and peptides. Deubiquitinating enzymes constitute a complex protein family involved in a multitude of cellular processes. The ubiquitin-specific proteases (UBP) are a group of enzymes whose predicted function is to reverse the ubiquitinating reaction by removing ubiquitin from a large variety of substrates. We have lately reported the characterization of human USP25, a specific-ubiquitin protease gene at 21q11.2, with a specific pattern of expression in murine fetal brains and adult testis. RESULTS: Database homology searches at the DNA and protein levels and cDNA library screenings led to the identification of a new UBP member in the human genome, named USP28, at 11q23. This novel gene showed preferential expression in heart and muscle. Moreover, cDNA, expressed sequence tag and RT-PCR analyses provided evidence for alternatively spliced products and tissue-specific isoforms. Concerning function, USP25 overexpression in Down syndrome fetal brains was shown by real-time PCR. CONCLUSIONS: On the basis of the genomic and protein sequence as well as the functional data, USP28 and USP25 establish a new subfamily of deubiquitinating enzymes. Both genes have alternatively spliced exons that could generate protein isoforms with distinct tissue-specific activity. The overexpression of USP25 in Down syndrome fetal brains supports the gene-dosage effects suggested for other UBP members related to aneuploidy syndromes.

Alternative Splicing↗

The muscle specific domain of mouse N-CAM: structure and alternative splicing patterns.

The neural cell adhesion molecule (N-CAM) is an important mediator of calcium independent cell-cell interactions. Variations in the primary structure of the protein are due to alternative splicing of pre-mRNA in the region encoding the extracellular, trans-membrane and cytoplasmic domains. In order to identify the patterns of exon usage during development of skeletal muscle and brain of the mouse, a coupled reverse-transcriptase/polymerase chain reaction was used to identify the murine homologues of the muscle-specific domain (MSD), located between exons 12 and 13 in human N-CAM mRNA. The cDNAs produced have been cloned and sequenced, or analysed directly. The amplification reactions were shown to maintain the concentration ratios of the initial cDNAs. The results indicate that the mouse homologue to exon MSD1a is under tissue and developmental regulation that is independent of exons MSD1b and MSD1c. The inclusion of the triplet exon AAG is also regulated in a cell- and stage-specific manner, which is independent of the other alternatively spliced exons of this domain.

Animals↗

Conserved expression of alternative splicing variants of peroxisomal acyl-CoA oxidase 1 in vertebrates and developmental and nutritional regulation in fish.

The acyl-coenzyme A oxidase 1 (ACOX1) catalyzes the first, rate-limiting step in peroxisomal beta-oxidation of medium to very long straight-chain fatty acids. Zebrafish (Danio rerio) acox1 was characterized and compared with homologs from other sequenced genomes, revealing a remarkable conservation of structure in the vertebrate lineage. Strictly conserved regions of the deduced proteins included acyl-CoA oxidase and FAD binding domains, as well as a COOH-terminal peroxisomal targeting signal. Whole mount in situ hybridization showed that zebrafish acox1 transcripts were diffusely distributed in early-stage embryonic cells, then discreetly expressed in the brain and widely present in the liver and intestine at later stages. An evolutionarily conserved alternative splicing of the corresponding acox1 primary transcript was identified in teleosts and tetrapods including mammals, giving rise, after exon skipping, to two splice variants, ACOX1-3I and ACOX1-3II. Real-time quantitative RT-PCR on zebrafish adult tissues indicated high levels of both variants in the liver, anterior intestine, and to a lesser extent, in the brain. However, the ACOX1-3II transcript variant was expressed seven times more in zebrafish brain than the ACOX1-3I variant. These data suggest a tissue-specific modulation of ACOX1 activity by exchanging exon 3 duplicated isoforms containing amino acid sequences that are potentially implicated in fatty acyl chain specificity. In addition, a significant pretranslational up-regulation of zebrafish and rainbow trout (Oncorhynchus mykiss) acox1 expression was observed in the anterior intestine after feeding. Taken together, these data indicate that ACOX1 alternative splicing isoforms play a key conserved role in the vertebrate fatty acid metabolism.

Acyl-CoA Oxidase↗

The alternatively spliced domain TnFnIII A1A2 of the extracellular matrix protein tenascin-C suppresses activation-induced T lymphocyte proliferation and cytokine production.

Several lines of evidences have suggested that T cell activation could be impaired in the tumor environment, a condition referred to as tumor-induced immunosuppression. We have previously shown that tenascin-C, an extracellular matrix protein highly expressed in the tumor stroma, inhibits T lymphocyte activation in vitro, raising the possibility that this molecule might contribute to tumor-induced immunosuppression in vivo. However, the region of the protein mediating this effect has remained elusive. Here we report the identification of the minimal region of tenascin-C that can inhibit T cell activation. Recombinant fragments corresponding to defined regions of the molecule were tested for their ability to inhibit in vitro activation of human peripheral blood T cells induced by anti-CD3 mAbs in combination with fibronectin or IL-2. A recombinant protein encompassing the alternatively spliced fibronectin type III domains of tenascin-C (TnFnIII A-D) vigorously inhibited both early and late lymphocyte activation events including activation-induced TCR/CD8 down-modulation, cytokine production, and DNA synthesis. In agreement with this, full length recombinant tenascin-C containing the alternatively spliced region suppressed T cell activation, whereas tenascin-C lacking this region did not. Using a series of smaller fragments and deletion mutants issued from this region, we have identified the TnFnIII A1A2 domain as the minimal region suppressing T cell activation. Single TnFnIII A1 or A2 domains were no longer inhibitory, while maximal inhibition required the presence of the TnFnIII A3 domain. Altogether, these data demonstrate that the TnFnIII A1A2 domain mediate the ability of tenascin-C to inhibit in vitro T cell activation and provide insights into the immunosuppressive activity of tenascin-C in vivo.

Alternative Splicing↗

Characterization and expression of alternatively spliced IgE heavy chain transcripts produced by peripheral blood lymphocytes.

We have investigated the IgE heavy chain isoforms produced in vivo by analyzing the epsilon mRNA species present in unstimulated PBL and expressing them individually in a myeloma cell line. Seven epsilon mRNA species were identified by using reverse transcription-PCR, cloning, and sequencing analysis. These species included the classical secreted (epsilon CH4-S) and membrane-bound (epsilon CH4-M1'-M2) IgE and five alternatively spliced epsilon transcripts. At the protein level, the five alternatively spliced epsilon transcripts (epsilon CH4*, epsilon CH4-M2', epsilon CH4'-1, epsilon CH4'-1-M2, and epsilon CH3-13-CH4) corresponded to four epsilon heavy chain isoforms, in which various parts of the CH4 domain were replaced by new stretches of amino acids at the carboxyl termini. The same epsilon mRNA species also were present in the IgE producing myeloma cell line U266. However, except for the classical membrane and secreted IgE, the corresponding proteins could not be identified. To further characterize the epsilon CH4-S, epsilon CH4*, epsilon CH4-M2', epsilon CH4'-1, and epsilon CH4-M1'-M2 species, we expressed them as chimeric mouse/human anti-4-hydroxy-5-iodo-3-nitrophenacetyl Abs in a mouse myeloma cell line. Only the classical secreted and membrane isoforms were found to be secreted or expressed on the cell surface, respectively, and the other forms were retained within the cells and degraded. These data suggest that some of the epsilon mRNA isoforms produced by PBL are aberrantly spliced mRNAs, the protein products of which are eliminated by post-translational events.

Amino Acid Sequence↗

The lymphoid transcription factor LyF-1 is encoded by specific, alternatively spliced mRNAs derived from the Ikaros gene.

The lymphocyte-specific DNA-binding protein LyF-1 interacts with a critical control element in the terminal deoxynucleotidyltransferase (TdT) promoter as well as with the promoters for other genes expressed during early stages of B- and T-cell development. We have purified LyF-1 and have obtained a partial amino acid sequence from proteolytic peptides. The amino acid sequence suggests that LyF-1 is a zinc finger protein encoded by the Ikaros gene, which previously was implicated in T-cell development. Recombinant Ikaros expressed in Escherichia coli bound to the TdT promoter, and antisera directed against the recombinant protein specifically blocked the DNA-binding activity of LyF-1 in crude extracts. Further analysis revealed that at least six distinct mRNAs are derived from the Ikaros/LyF-1 gene by alternative splicing. Only two of the isoforms possess the N-terminal zinc finger domain that is necessary and sufficient for TdT promoter binding. Although both of these isoforms bound to similar sequences in the TdT, lambda 5, VpreB, and lck promoters, one isoform contains an additional zinc finger that resulted in altered recognition of some binding sites. At least four of the Ikaros/LyF-1 isoforms were detectable in extracts from B- and T-cell lines, with the relative amounts of the isoforms varying considerably. These data reveal that the LyF-1 protein is encoded by specific mRNAs derived from the alternatively-spliced Ikaros gene, suggesting that this gene may be important for the early stages of both B- and T-lymphocyte development.

Alternative Splicing↗

Disease-associated mutations and alternative splicing alter the enzymatic and motile activity of nonmuscle myosins II-B and II-C.

Human families with single amino acid mutations in nonmuscle myosin heavy chain (NMHC) II-A (MYH9) and II-C (MYH14) have been described as have mice generated with a point mutation in NMHC II-B (MYH10). These mutations (R702C and N93K in human NMHC II-A, R709C in murine NMHC II-B, and R726S in human NMHC II-C) result in phenotypes affecting kidneys, platelets, and leukocytes (II-A), heart and brain (II-B), and the inner ear (II-C). To better understand the mechanisms underlying these defects, we characterized the in vitro activity of mutated and wild-type baculovirus-expressed heavy meromyosin (HMM) II-B and II-C. We also expressed two alternatively spliced isoforms of NMHC II-C which differ by inclusion/exclusion of eight amino acids in loop 1, with and without mutations. Comparison of the actin-activated MgATPase activity and in vitro motility shows that mutation of residues Asn-97 and Arg-709 in HMM II-B and the homologous residue Arg-722 (Arg-730 in the alternatively spliced isoform) in HMM II-C decreases both parameters but affects in vitro motility more severely. Analysis of the transient kinetics of the HMM II-B R709C mutant shows an extremely tight affinity of HMM for ADP and a very slow release of ADP from acto-HMM. Although mutations generally decreased HMM activity, the R730S mutation in HMM II-C, unlike the R730C mutation, had no effect on actin-activated MgATPase activity but decreased the rate of in vitro motility by 75% compared with wild type. Insertion of eight amino acids into the HMM II-C heavy chain increases both actin-activated MgATPase activity and in vitro motility.

Actins↗

Structure and expression of H-type GDP-L-fucose:beta-D-galactoside 2-alpha-L-fucosyltransferase gene (FUT1). Two transcription start sites and alternative splicing generate several forms of FUT1 mRNA.

The expression of the ABO antigens on erythrocyte membranes is regulated by H gene (FUT1)-encoded alpha(1,2)fucosyltransferase activity. We have examined the expression of the FUT1 in several tumor cell lines, including erythroid lineage and normal bone marrow cells, by Northern blot and/or reverse transcription-polymerase chain reaction (RT-PCR) analyses. RT-PCR indicated that bone marrow cells, erythroleukemic cells (HEL), and highly undifferentiated leukemic cells (K562) that have erythroid characteristics expressed the FUT1 mRNA while four leukemic cell lines did not. The FUT1 mRNA was also demonstrated in gastric, colonic, and ovarian (MCAS) cancer cell lines by RT-PCR. Northern blot analysis indicated that a 4. 0-kilobase FUT1 transcript was expressed in some of these tumor cell lines. Rapid amplification of 5' cDNA end (RACE) analysis suggested that the FUT1 transcript had several forms generated by two distinct transcription start sites and alternative splicing. The results of RT-PCR using specific primers for each starting exon suggested that two transcription initiation sites (exon 1A and exon 2A) of the FUT1 were identified in gastric cancer cells and in ovarian cancer cells. Only exon 1A was identified as a transcription start site in another gastric cancer cell line, two colonic cancer cell lines, and in K562 cells, whereas only exon 2A was identified in HEL cells and in bone marrow cells. These two transcription start sites were located 1.8 kilobases apart. Therefore, two distinct promoters appeared to be present in the FUT1. The distinct promoters of the FUT1 and alternative splicing of the FUT1 mRNA may be associated with time- and tissue-specific expression of the FUT1.

Alternative Splicing↗

Complete structure of the murine p36 (annexin II) gene. Identification of mRNAs for both the murine and the human gene with alternatively spliced 5' noncoding exons.

p36 (also termed annexin II) is a 39 kDa Ca2+/phospholipid-binding, membrane-associated protein that is a protein-tyrosine kinase substrate. We report here studies of the noncoding exons of p36, which combined with our earlier studies of the coding exons, allow us to conclude that the murine p36 gene is 34 kb in length with 14 exons. Comparison of the genes coding for mouse and human p36 (annexin II) and mouse, rat and human p35 (annexin I) and pigeon cp35 (an annexin I-related protein) shows strong genomic structural conservation supporting the hypothesis that these genes had a common ancestor. Both human and murine p36 mRNAs were found to be alternatively spliced in their 5' noncoding region. In both cases exon 2 is a cassette exon, which is present in a small fraction of p36 mRNAs. In type 1 mouse p36 mRNA the first noncoding 44 base exon 1 is joined to exon 3, the first of the 12 coding exons. In type 2 mRNA a 70 base noncoding exon (exon 2) is inserted between exon 1 and exon 3. Type 1 mRNA was present in all cell types studied as revealed by Northern analysis and primer extension, whereas type 2 mRNA could only be detected by RACE or PCR, indicating that it is of very low abundance. The major transcription start site of the mouse p36 gene was mapped by primer extension to be 61 bp upstream of the AUG initiation codon, which corresponds to type 1 mRNA, The murine p36 gene enhancer/promoter region contains a putative TATA box and several other potential regulatory sequences. The two alternatively-spliced human p36 mRNAs differ by the presence or absence of a noncoding 81 base exon (exon 2) inserted after exon 1, with exon 2-containing mRNAs representing approximately 10% of total p36 mRNA. The 300 bp spanning the promoter and exons 1-3 of the human and murine p36 genes show strong sequence homology immediately before and after the major transcription start site except in the region corresponding to exon 2, where homology is more limited.

3T3 Cells↗

MAGI-1: a widely expressed, alternatively spliced tight junction protein.

Tight junctions are apically localized structures that regulate the passage of small molecules and proteins through intercellular regions of epithelial or endothelial cells. These structures are complex multimolecular assemblages that contain both transmembrane and membrane-associated proteins. MAGUKs (Membrane-Associated Guanylate Kinases) are a family of scaffolding proteins that contain multiple protein interaction domains, including PDZ, SH3, WW, and guanylate kinase motifs, and have been grouped into five discrete subfamilies based on homology. Little is known regarding the most recently described subfamily of MAGUKs, termed MAGIs (MAGUKS with Inverted domain structure). Here we show that two of the three known MAGI isoforms, MAGI-1 and MAGI-3, are present in the tight junctions of cultured epithelial cells. A broader examination of MAGI-1 expression in vivo shows that it is present in the tight junctions of all epithelial cell types examined. Human MAGI-1 transcripts are alternatively spliced at three sites, and two forms are expressed only in nonepithelial tissues, predominantly in brain. The major form that is expressed in cultured colon carcinoma epithelial cells, as well as several epithelial-rich tissues, contains an extended carboxy terminus encoding potential nuclear targeting signals. MAGI-1, ZO-1, and ZO-2 all colocalize in nonpolarized epithelial cells, suggesting that they form a preassembled complex that is incorporated into the tight junction upon polarization. Finally, all of the alternatively spliced forms of MAGI-1 show tight junction localization, and this localization occurs in the absence of the guanylate kinase and WW domains as well as the extended carboxy terminus.

Alternative Splicing↗

Identification of an alternatively spliced RNA for the Ras suppressor RSU-1 in human gliomas.

Previous studies demonstrated that the Ras suppressor, RSU-1, localizes to human chromosome 10p13, a region frequently deleted in high grade gliomas, and that RSU-1 expression inhibited the tumorigenesis of a glioblastoma cell line. We have now examined RNA from human glial tumors for RSU-1 expression by RT-PCR using primers for the 5' and 3' ends of the RSU-1 open reading frame. Analysis of the amplified RSU-1 cDNA demonstrated that in addition to the entire 858 bp RSU-1 open reading frame, a shorter 725 bp RSU-1 fragment was amplified as well. Sequencing of this product revealed that it encoded a RSU-1 cDNA product which was missing a single 133 bp internal exon. This exon-deleted product was found in 30% of the high grade gliomas studied and 2/3 oligodendrogliomas, but not in other CNS tumors, bladder or colon tumors or normal tissue. The exon-deleted RSU-1 product was infrequently detected in RNA from human tumor cell lines. Expression of an HA-tagged form of the deleted RSU-1 protein in transfected Cos 1 cells revealed that the protein was unstable, with a half life of less than 1 h, in contrast to the full length HA-tagged Rsu-1 protein which was stable for more than 4 h. These results suggest that the alternative splicing of the RSU-1 RNA to produce the exon-deleted form constitutes a mechanism for reduction or loss of functional Rsu-1. Because the expression of Rsu-1 can inhibit malignant growth of glioblastoma cells, the depletion of Rsu-1, via the production of the alternatively spliced form of RSU-1, may inhibit growth regulation in tumors.

Alternative Splicing↗

Alternative splicing of the APC gene and its association with terminal differentiation.

The human tumor suppressor gene, APC, is composed of at least 21 exons, 7 of which are alternatively expressed. Sixteen APC transcripts that differ in their 5'-most regions and arise by the alternative inclusion of 6 of these exons have been identified by reverse transcription-PCR analysis of RNA prepared from human, mouse, and rat cell lines and tissues. Tissue-specific differences were observed in the expression of APC transcripts without exon 1, a coding region for a heptad repeat that supports APC homodimerization. Transcripts without exon 1 were observed at high levels in postmitotic, differentiated tissues and in two cell lines following the induction of differentiation. Sequence analysis of these novel open reading frames predicts APC proteins with different amino-terminal domains and therefore potentially different abilities to associate with other proteins. Our findings suggest that the alternative splicing of APC leads to alternative forms of APC proteins with potentially unique functions in growth control and differentiation.

Alternative Splicing↗

A previously unidentified alternatively spliced isoform of t(8;21) transcript promotes leukemogenesis.

The t(8;21)(q22;q22) translocation is one of the most common genetic abnormalities in acute myeloid leukemia (AML), identified in 15% of all cases of AML, including 40-50% of FAB M2 subtype and rare cases of M0, M1 and M4 subtypes. The most commonly known AML1-ETO fusion protein (full-length AML1-ETO) from this translocation has 752 amino acids and contains the N-terminal portion of RUNX1 (also known as AML1, CBFalpha2 or PEBP2alphaB), including its DNA binding domain, and almost the entire RUNX1T1 (also known as MTG8 or ETO) protein. Although alterations of gene expression and hematopoietic cell proliferation have been reported in the presence of AML1-ETO, its expression does not lead to the development of leukemia. Here, we report the identification of a previously unknown alternatively spliced isoform of the AML1-ETO transcript, AML1-ETO9a, that includes an extra exon, exon 9a, of the ETO gene. AML1-ETO9a encodes a C-terminally truncated AML1-ETO protein of 575 amino acids. Expression of AML1-ETO9a leads to rapid development of leukemia in a mouse retroviral transduction-transplantation model. More importantly, coexpression of AML1-ETO and AML1-ETO9a results in the substantially earlier onset of AML and blocks myeloid cell differentiation at a more immature stage. These results indicate that fusion proteins from alternatively spliced isoforms of a chromosomal translocation may work together to induce cancer development.

Alternative Splicing↗

Phytochelatin synthases of the model legume Lotus japonicus. A small multigene family with differential response to cadmium and alternatively spliced variants.

The biosynthesis of phytochelatins and homophytochelatins has been studied in nodulated plants of the model legume Lotus (Lotus japonicus). In the first 6 to 24 h of treatment with cadmium (Cd), roots started to synthesize elevated amounts of both polypeptides, with a concomitant increase of glutathione and a decrease of homoglutathione, indicating the presence of active phytochelatin synthase (PCS) genes. Screening of transformation-competent artificial chromosome libraries allowed identification of a cluster of three genes, LjPCS1, LjPCS2, and LjPCS3, which were mapped at 69.0 cM on chromosome 1. The genes differ in exon-intron composition and responsiveness to Cd. Gene structures and phylogenetic analysis of the three protein products, LjPCS1-8R, LjPCS2-7N, and LjPCS3-7N, are consistent with two sequential gene duplication events during evolution of vascular plants. Two sites for alternative splicing in the primary transcripts were identified. One of them, involving intron 2 of the LjPCS2 gene, was confirmed by the finding of the two predicted mRNAs, encoding LjPCS2-7R in roots and LjPCS2-7N in nodules. The amino acid sequences of LjPCS2-7R (or LjPCS2-7N) and LjPCS3-7N share 90% identity, but have only 43% to 59% identity with respect to the typical PCS1 enzymes of Lotus and other plants. The unusual LjPCS2-7N and LjPCS3-7N proteins conferred Cd tolerance when expressed in yeast (Saccharomyces cerevisiae) cells, whereas the alternatively spliced form, LjPCS2-7R, differing only in a five-amino acid motif (GRKWK) did not. These results unveil complex regulatory mechanisms of PCS expression in legume tissues in response to heavy metals and probably to other developmental and environmental factors.

Alternative Splicing↗

Alternative splicing of dystrobrevin regulates the stoichiometry of syntrophin binding to the dystrophin protein complex.

Dystrophin coordinates the assembly of a complex of structural and signalling proteins that is required for normal muscle function. A key component of the dystrophin-associated protein complex (DPC) is alpha-dystrobrevin, a dystrophin-related and -associated protein whose absence results in muscular dystrophy and neuromuscular junction defects [1,2]. The current model of the DPC predicts that dystrophin and dystrobrevin each bind a single syntrophin molecule [3]. The syntrophins are PDZ-domain-containing proteins that facilitate the recruitment of signalling proteins such as nNOS (neuronal nitric oxide synthase) to the DPC [4]. Here we show, using yeast two-hybrid analysis and biochemical binding studies, that alpha-dystrobrevin in fact contains two independent syntrophin-binding sites in tandem. The previously undescribed binding site is situated within an alternatively spliced exon of alpha-dystrobrevin, termed the variable region-3 (vr3) sequence, which is specifically expressed in skeletal and cardiac muscle [5,6]. Analysis of the syntrophin-binding region of dystrobrevin reveals a tandem pair of predicted alpha helices with significant sequence similarity. These alpha helices, each termed a syntrophin-binding motif, are also highly conserved in dystrophin and utrophin. Together these data show that there are four potential syntrophin-binding sites per dystrophin complex in skeletal muscle: two on dystrobrevin and two on dystrophin or utrophin. Furthermore, alternative splicing of dystrobrevin provides a mechanism for regulating the stoichiometry of syntrophin association with the DPC. This is likely to have important consequences for the recruitment of specific signalling molecules to the DPC and ultimately for its function.

Alternative Splicing↗

The MUC3 gene encodes a transmembrane mucin and is alternatively spliced.

Epithelial mucins are a family of secreted and cell surface glycoproteins expressed by epithelial tissues and implicated in epithelial cell protection, adhesion modulation and signaling. The gene encoding human MUC3 (hMUC3), localised to chromosome 7q22, is most highly expressed in the small intestine. It has previously been reported to be a non-transmembrane mucin with minimal homology to its suggested orthologues from rat (rMuc3) and mouse (mMuc3). RT-PCR was performed to investigate the carboxyl terminus of the published sequence of hMUC3 from normal colon and small intestine tissues and also from a series of 10 colorectal cancer cell lines. Two distinct PCR products were identified. In contrast to the previously published hMUC3 sequence, which terminates shortly after a single cysteine-rich EGF-like domain, conceptual protein translation of the dominant and largest PCR product identified two extracellular cysteine-rich EGF-like domains separated by an N-glycosylation-rich domain and a potential coiled-coil region, followed by a putative transmembrane region and a 75 amino acid cytoplasmic tail. The smaller of the two PCR products was found to be an alternative splice variant of MUC3 including the first EGF-like domain but lacking part of the second EGF-like domain and the transmembrane region. Nine out of 10 colorectal cancer cell lines were found to express MUC3. Interestingly, one of the cell lines, LoVo, expressed predominantly the alternative splice form lacking a transmembrane domain. Structural homology of the new protein sequence of hMUC3 with rMuc3 and mMuc3 indicates it is closely related to the rodent proteins and is likely to be involved in ligand-binding and intracellular signaling. The new finding that MUC3 encodes a transmembrane molecule presents a new paradigm for the structure of this mucin and the manner in which it may function.

Alternative Splicing↗