Biomedical subjects
Publications and source records attributed to M E Hemler.
Relatively little attention has been given to the large family of abundantly expressed transmembrane proteins known as tetraspanins. Now, the importance of tetraspanins is strongly supported by emerging genetic evidence, coupled with new insights into the biochemistry and functions of tetraspanin protein complexes.
BACKGROUND: The CD98 (4F2, FRP-1) is a widely expressed cell surface protein heterodimer composed of a glycosylated heavy chain and a non-glycosylated light chain. Originally described as a T cell activation antigen, it was later shown to function in amino acid transport, cell fusion and homotypic cell aggregation. Several lines of evidence suggest its functional interaction with integrins but the biochemical basis for this interaction has been unclear. RESULTS: We demonstrate that CD98 constitutively and specifically associates with beta1 integrins (alpha2beta1,alpha3beta1, alpha5beta1 and alpha6beta1), but minimally with alpha4beta1. Integrin-CD98 association was established by reciprocal immunoprecipitation experiments, and confirmed by CD98-induced clustering of alpha3beta1 but not alpha4beta1 on the surface of rhabdomyosarcoma cells. Integrin-CD98 association is independent of the alpha subunit cytoplasmic tail, is maintained in alpha3beta1 ligand-interaction deficient mutants, and is not inhibited by EDTA. Within the CD98 heavy chain, a C109S mutation (but not a C330S mutation) caused a loss of beta1 integrin association. The same C109S mutation also caused a loss of CD98 light chain association. Importantly, CD98 associated selectively with beta1 integrins present in low density "light membrane" fractions on a sucrose gradient. CD98 was not present in dense fractions that contained the majority of beta1 integrins. Notably, the C109S mutant of CD98, that did not associate with beta1 integrins, showed also a reduced localization into light membrane fractions. CONCLUSIONS: We demonstrate that CD98 association with beta1 integrins is specific, occurs in the context of low density membranes, and may require the CD98 light chain.
A novel Ig superfamily protein, EWI-2, was co-purified with tetraspanin protein CD81 under relatively stringent Brij 96 detergent conditions and identified by mass spectrometric protein sequencing. EWI-2 associated specifically with CD9 and CD81 but not with other tetraspanins or with integrins. Immunodepletion experiments indicated that EWI-2-CD9/CD81 interactions are highly stoichiometric, with approximately 70% of CD9 and CD81 associated with EWI-2 in an embryonic kidney cell line. The EWI-2 molecule was covalently cross-linked (in separate complexes) to both CD81 and CD9, suggesting that association is direct. EWI-2 is part of a novel Ig subfamily that includes EWI-F (F2alpha receptor regulatory protein (FPRP), CD9P-1), EWI-3 (IgSF3), and EWI-101 (CD101). All four members of this Ig subfamily contain a Glu-Trp-Ile (EWI) motif not seen in other Ig proteins. As shown previously, the EWI-F molecule likewise forms highly proximal, specific, and stoichiometric complexes with CD9 and CD81. Human and murine EWI-2 protein sequences are 91% identical, and transcripts in the two species are expressed in virtually every tissue tested. Thus, EWI-2 potentially contributes to a variety of CD9 and CD81 functions seen in different cell and tissue types.
Translocation of conventional protein kinases C (PKCs) to the plasma membrane leads to their specific association with transmembrane-4 superfamily (TM4SF; tetraspanin) proteins (CD9, CD53, CD81, CD82, and CD151), as demonstrated by reciprocal co-immunoprecipitation and covalent cross-linking experiments. Although formation and maintenance of TM4SF-PKC complexes are not dependent on integrins, TM4SF proteins can act as linker molecules, recruiting PKC into proximity with specific integrins. Previous studies showed that the extracellular large loop of TM4SF proteins determines integrin associations. In contrast, specificity for PKC association probably resides within cytoplasmic tails or the first two transmembrane domains of TM4SF proteins, as seen from studies with chimeric CD9 molecules. Consistent with a TM4SF linker function, only those integrins (alpha(3)beta(1), alpha(6)beta(1), and a chimeric "X3TC5" alpha(3) mutant) that associated strongly with tetraspanins were found in association with PKC. We propose that PKC-TM4SF-integrin structures represent a novel type of signaling complex. The simultaneous binding of TM4SF proteins to the extracellular domains of the integrin alpha(3) subunit and to intracellular PKC helps to explain why the integrin alpha3 extracellular domain is needed for both intracellular PKC recruitment and PKC-dependent phosphorylation of the alpha(3) integrin cytoplasmic tail.
Extracellular matrix metalloproteinase inducer (EMMPRIN; CD147) is a heavily glycosylated protein containing two immunoglobulin superfamily domains. It is enriched on the surface of tumor cells and stimulates the production of matrix metalloproteinases (MMPs) by adjacent stromal cells. Here we use CD147 transfectants and immobilized recombinant CD147-Fc fusion protein to show that CD147/FMMPRIN engages in a homophilic interaction, predominantly through the first immunoglobulin domain. Anti-CD147 antibody 8G6 and recombinant CD147-Fc fusion protein markedly inhibited not only homophilic interaction, but also the production of secreted MMP-2 by breast cancer cell line MDA-435 and the MMP-2-dependent invasion of MDA-435 cells through reconstituted basement-membrane Matrigel. Purified native CD147 induced the production of secreted MMP not only by dermal fibroblasts (MMP-1) but also by MDA-435 cells themselves (MMP-2), suggesting homophilic CD147-binding may occur in the context of both heterotypic and homotypic cell-cell interactions. Purified deglycosylated CD147 failed to induce MMP-1 or MMP-2, but instead antagonized the MMP-1-inducing activity of purified native CD147. Our results suggest that homophilic CD147 interactions may play a key role in MMP-2 production and tumor cell invasion, and that perturbation of this molecule may have potential therapeutic uses in the prevention of MMP-2 and MMP-1-dependent cancer metastasis.
In carcinoma cells, the beta 4 integrin functions in a ligand-independent manner to promote proliferation, migration, and invasion. An interesting new paper describes a mechanism whereby the beta 4 integrin cytoplasmic tail becomes an integrin ligand-independent adaptor protein for the Met receptor tyrosine kinase, thereby enhancing the mitogenic, morphogenic, and motogenic properties of Met.
Integrin alpha 3A cytoplasmic tail phosphorylation was mapped to amino acid S1042, as determined by mass spectrometry, and confirmed by mutagenesis. This residue occurs within a "QPSXXE" motif conserved in multiple alpha chains (alpha 3A, alpha 6A, alpha 7A), from multiple species. Phosphorylation of alpha 3A and alpha 6A did not appear to be directly mediated by protein kinase C (PKC) alpha, beta, gamma, delta, epsilon, zeta, or mu, or by any of several other known serine kinases, although PKC has an indirect role in promoting phosphorylation. A S1042A mutation did not affect alpha 3-Chinese hamster ovary (CHO) cell adhesion to laminin-5, but did alter 1) alpha 3-dependent tyrosine phosphorylation of focal adhesion kinase and paxillin (in the presence or absence of phorbol 12-myristate 13 acetate stimulation), and p130(CAS) (in the absence of phorbol 12-myristate 13 acetate stimulation), 2) the shape of cells spread on laminin-5, and 3) alpha 3-dependent random CHO cell migration on laminin-5. In addition, S1042A mutation altered the PKC-dependent, ligand-dependent subcellular distribution of alpha 3 and F-actin in CHO cells. Together, the results demonstrate clearly that alpha 3A phosphorylation is functionally relevant. In addition, the results strongly suggest that alpha 3 phosphorylation may regulate alpha 3 integrin interaction with the cytoskeleton.
Recent literature suggests that tetraspanin proteins (transmembrane 4 superfamily; TM4SF proteins) may associate with each other and with many other transmembrane proteins to form large complexes that sometimes may be found in lipid rafts. Here we show that prototype complexes of CD9 or CD81 (TM4SF proteins) with alpha(3)beta(1) (an integrin) and complexes of CD63 (a TM4SF protein) with phosphatidylinositol 4-kinase (PtdIns 4-K) may indeed localize within lipid raft-like microdomains, as seen by three different criteria. First, these complexes localize to low density light membrane fractions in sucrose gradients. Second, CD9 and alpha(3) integrin colocalized with ganglioside GM1 as seen by double staining of fixed cells. Third, CD9-alpha3beta1 and CD81-alpha3beta1 complexes were shifted to a higher density upon cholesterol depletion from intact cells or cell lysate. However, CD9-alpha3beta1, CD81-alpha3beta1, and CD63-PtdIns 4-K complex formation itself was not dependent on localization into raftlike lipid microdomains. These complexes did not require cholesterol for stabilization, were maintained within well solubilized dense fractions from sucrose gradients, were stable at 37 degrees C, and were small enough to be included within CL6B gel filtration columns. In summary, prototype TM4SF protein complexes (CD9-alpha3beta1, CD81-alpha3beta1, and CD63-PtdIns 4-K) can be solubilized as discrete units, independent of lipid microdomains, although they do associate with microdomains resembling lipid rafts.
CD81 and CD9, members of the transmembrane-4 superfamily (TM4SF; tetraspanins), form extensive complexes with other TM4SF proteins, integrins, and other proteins, especially in mild detergents. In moderately stringent Brij 96 lysis conditions, CD81 and CD9 complexes are virtually identical to each other, but clearly distinct from other TM4SF complexes. One of the most prominent proteins within CD81 and CD9 complexes is identified here as FPRP, the 133-kDa prostaglandin F(2alpha) receptor regulatory protein. FPRP, a cell-surface Ig superfamily protein, associates specifically with CD81 or with CD81 and CD9, but not with integrins or other TM4SF proteins. In contrast to other CD81- and CD9-associating proteins, FPRP associates at very high stoichiometry, with essentially 100% of cell-surface FPRP on 293 cells being CD81- and CD9-associated. Also, CD81.CD9.FPRP complexes have a discrete size (<4 x 10(6) Da) as measured by gel permeation chromatography and remain intact after disruption of cholesterol-rich membrane microdomains by methyl-beta-cyclodextrin. Although CD81 associated with both alpha(3) integrin and FPRP in 293 cells, the alpha(3)beta(1).CD81 and CD81.CD9.FPRP complexes were distinct, as determined by immunoprecipitation and immunodepletion experiments. In conclusion, our data affirm the existence of distinct TM4SF complexes with unique compositions and specifically characterize FPRP as the most robust, highly stoichiometric CD81- and/or CD9-associated protein yet described.
In earlier work we established that phosphoinositide 4-kinase (PI 4-kinase) may associate with transmembrane 4 superfamily (TM4SF, tetraspanin) proteins, but critical specificity issues were not addressed. Here we demonstrate that at least five different TM4SF proteins (CD9, CD63, CD81, CD151 and A15/TALLA1) can associate with a similar or identical 55 kDa type II PI 4-kinase. These associations were specific, since we found no evidence for other phosphoinositide kinases (e.g. phosphoinositide 3-kinase and phosphoinositide-4-phosphate 5-kinase) associating with TM4SF proteins, and many other TM4SF proteins (including CD82 and CD53) did not associate with PI 4-kinase. CD63-PI 4-kinase complexes were almost entirely intracellular, and thus are distinct from other TM4SF-PI 4-kinase complexes (e.g. involving CD9), which are largely located in the plasma membrane. These results suggest that a specific subset of TM4SF proteins may recruit PI 4-kinase to specific membrane locations, and thereby influence phosphoinositide-dependent signalling.
Previously we established that the alpha(3)beta(1) integrin shows stable, specific, and stoichiometric association with the TM4SF (tetraspannin) protein CD151. Here we used a membrane impermeable cross-linking agent to show a direct association between extracellular domains of alpha(3)beta(1) and CD151. The alpha(3)beta(1)-CD151 association site was then mapped using chimeric alpha(6)/alpha(3) integrins and CD151/NAG2 TM4SF proteins. Complex formation required an extracellular alpha(3) site (amino acids (aa) 570-705) not previously known to be involved in specific integrin contacts with other proteins and a region (aa 186-217) within the large extracellular loop of CD151. Notably, the anti-CD151 monoclonal antibody TS151r binding epitope, previously implicated in alpha(3) integrin association, was mapped to the same region of CD151 (aa 186-217). Finally, we demonstrated that both NH(2)- and COOH-terminal domains of CD151 are located on the inside of the plasma membrane, thus confirming a long suspected model of TM4SF protein topology.
Proteins in the transmembrane-4-superfamily (TM4SF) form many different complexes with proteins in the integrin family, but the functional utility of these complexes has not yet been demonstrated. Here we show that TM4SF proteins CD151, CD81, and CD63 co-distribute with alpha3beta1 integrin on neurites and growth cones of human NT2N cells. Also, stable CD151-alpha3beta1 and CD81-alpha3beta1 complexes were recovered in NT2N detergent lysates. Total NT2N neurite outgrowth on laminin-5 (a ligand for alpha3beta1 integrin) was strongly inhibited by anti-CD151 and -CD81 antibodies either together ( approximately 85% inhibition) or alone ( approximately 45% inhibition). Notably, these antibodies had no inhibitory effect on NT2N neurites formed on laminin-1 or fibronectin, when alpha3beta1integrin was not engaged. Neurite number, length, and rate of extension were all affected by anti-TM4SF antibodies. In summary: (1) these substrate-dependent inhibition results strongly suggest that CD151 and CD81 associations with alpha3beta1 are functionally relevant, (2) TM4SF proteins CD151 and CD81 make a strong positive contribution toward neurite number, length, and rate of outgrowth, and (3) NT2N cells, a well-established model of immature central nervous system neurons, can be a powerful system for studies of integrin function in neurite outgrowth and growth cone motility.
The role of transmembrane 4 superfamily (TM4SF) proteins during muscle cell fusion has not been investigated previously. Here we show that the appearance of TM4SF protein, CD9, and the formation of CD9-beta1 integrin complexes were both regulated in coordination with murine C2C12 myoblast cell differentiation. Also, anti-CD9 and anti-CD81 monoclonal antibodies substantially inhibited and delayed conversion of C2C12 cells to elongated myotubes, without affecting muscle-specific protein expression. Studies of the human myoblast-derived RD sarcoma cell line further demonstrated that TM4SF proteins have a role during muscle cell fusion. Ectopic expression of CD9 caused a four- to eightfold increase in RD cell syncytia formation, whereas anti-CD9 and anti-CD81 antibodies markedly delayed RD syncytia formation. Finally, anti-CD9 and anti-CD81 monoclonal antibodies triggered apoptotic degeneration of C2C12 cell myotubes after they were formed. In summary, TM4SF proteins such as CD9 and CD81 appear to promote muscle cell fusion and support myotube maintenance.
In a yeast two-hybrid screen, a protein named ICAP-1 (beta1 integrin cytoplasmic domain associated protein) associated with the integrin beta1 cytoplasmic tail but not with tails from three other integrin beta subunits (beta2, beta3, and beta5) or from seven different alpha subunits. Likewise in human cells, ICAP-1 associated specifically with the beta1 but not beta2, beta3, or beta5 tails. The carboxyl-terminal 14 amino acids of beta1 were critical for ICAP-1 interaction. ICAP-1 is a ubiquitously expressed protein of 27 and 31 kDa, with the smaller form being preferentially solubilized by Triton X-100. Phosphorylation of both 27- and 31-kDa forms was constitutive but was increased by 1.5-2-fold upon cell spreading on fibronectin, compared with poly-L-lysine. Also, ICAP-1 contributes to beta1 integrin-dependent migration because (i) ICAP-1 transfection markedly increased chemotactic migration of COS7 cells through fibronectin-coated but not vitronectin-coated porous filters, and (ii) support of beta1-dependent cell migration (in Chinese hamster ovary cells transfected with various wild type and mutant beta1 forms) correlated with ICAP-1 association. In summary, ICAP-1 (i) associates specifically with beta1 integrins, (ii) is phosphorylated upon beta1 integrin-mediated adhesion, and (iii) may regulate beta1-dependent cell migration.
Rho family GTPases regulate diverse cellular processes, including extracellular signal-mediated actin cytoskeleton reorganization and cell growth. The functions of GTPases are positively regulated by guanine nucleotide exchange factors, which promote the exchange of GDP for GTP. Trio is a complex protein possessing two guanine nucleotide exchange factor domains, each with adjacent pleckstrin homology and SH3 domains, a protein serine/threonine kinase domain with an adjacent immunoglobulin-like domain and multiple spectrin-like domains. To assess the functional role of the two Trio guanine nucleotide exchange factor domains, NIH 3T3 cell lines stably expressing the individual guanine nucleotide exchange factor domains were established and characterized. Expression of the amino-terminal guanine nucleotide exchange factor domain results in prominent membrane ruffling, whereas cells expressing the carboxy-terminal guanine nucleotide exchange factor domain have lamellae that terminate in miniruffles. Moreover, cells expressing the amino-terminal guanine nucleotide exchange factor domain display more rapid cell spreading, haptotactic cell migration and anchorage-independent growth, suggesting that Trio regulates both cell motility and cell growth. Expression of full-length Trio in COS cells also alters actin cytoskeleton organization, as well as the distribution of focal contact sites. These findings support a role for Trio as a multifunctional protein that integrates and amplifies signals involved in coordinating actin remodeling, which is necessary for cell migration and growth.
The 80/40-kDa CD98 protein complex was purified using an anti-CD98 heavy chain monoclonal antibody coupled to Sepharose beads. Eluted proteins were subjected to preparative SDS-polyacrylamide gel electrophoresis, and protein corresponding to the 40-kDa CD98 light chain was excised. Following proteolysis with trypsin, a peptide fragment was sequenced by mass spectrometry. The nine residues obtained were identical to established C-terminal sequences of the human E16 and rat TA1 proteins, suggesting that TA1/E16 protein is the CD98 light chain. Consistent with this, anti-TA1/E16 antibodies specifically immunoblotted the approximately 35-40-kDa light chain present upon immunoprecipitation of the human CD98 complex. Furthermore, anti-CD98 heavy chain antibody specifically co-immunoprecipitated hemagglutinin-tagged light chain from cells transfected with hemagglutinin-tagged E16 cDNA. In conclusion, the CD98 light chain is identical to the TA1/E16 protein, based on partial amino acid sequence identity, antibody cross-reactivity, genetic reconstitution evidence, similar molecular size, and comparable cell distribution.
The anti-integrin beta1 MAb 15/7 sometimes may be a reporter of integrin activation or ligand occupancy. However, certain beta1 tail deletions eliminate ligand binding despite inducing maximal constitutive 15/7 expression [Puzon-Mclaughlin et al. (1 996): J Biol Chem 271:16580-16585]. Here we describe beta1 tail mutations (e.g., double point mutations [D759L/F763L, F766L/E769L], or replacement of the beta1 tail by the beta5 tail) that prevent rather than induce constitutive appearance of the 15/7 epitope. Despite variable losses of constitutive 15/7 epitope, these mutants all retained a similar inducible 15/7 epitope component as seen upon incubation with GRGDSP peptide ligand. In addition, constitutive 15/7 expression did not correlate with integrin localization into focal adhesions. In conclusion, we show for the first time for a fully functional integrin that specific mutations within the beta1 tail can down-regulate the constitutive appearance of an extracellular conformation defined by MAb 15/7. Because this regulation occurs away from the ligand binding site and does not correlate with responsiveness to integrin ligand, cell adhesion, or localization into focal adhesions, a novel type of conformational regulation is suggested.