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PubMed1994

The IgM antigen receptor of B lymphocytes is associated with prohibitin and a prohibitin-related protein.

The two major classes of antigen receptors on murine B lymphocytes, mIgM and mIgD, are both contained in a complex with two additional molecules, Ig-alpha and Ig-beta, which permit signal transduction. Accordingly, early biochemical events after antigen binding to either receptor are similar; biological effects, however, are different. Here, we describe three newly discovered intracellular proteins of 32, 37 and 41 kDa molecular mass, that are non-covalently associated with mIgM, but not with mIgD. These proteins coprecipitate with mIgM in Triton X-100 and Nonidet P-40, but not in digitonin lysates. In addition, mIgM is to some extent associated with 29 and 31 kDa proteins that are predominantly associated with mIgD (see accompanying paper). Amino acid sequencing of p32 and p37 identified p32 as mouse prohibitin; this was corroborated by Western blot analysis with antibodies specific for rat prohibitin. p37 is a newly discovered protein. cDNA clones for both proteins were isolated and sequenced. The deduced amino acid sequence of p32 is identical to that of rat prohibitin. p37 is highly homologous to p32. Since prohibitin was identified as an inhibitor of cell proliferation, its association with mIgM, but not mIgD, could explain the different biological events elicited after engagement of each receptor.

Amino Acid Sequence↗

PubMed2007

Differential regulation of human YY1 and caspase 7 promoters by prohibitin through E2F1 and p53 binding sites.

Prohibitin is a 30 kDa growth suppressive protein that has pleiotropic functions in the cell. Although prohibitin has been demonstrated to have potent transcriptional regulatory functions, it has also been proposed to facilitate protein folding in the mitochondria and promote cell migration in association with Raf-1. Our previous studies have shown that prohibitin physically interacts with the marked-box domain of E2F family members and represses their transcriptional activity; in contrast, prohibitin could bind to and enhance the transcriptional activity of p53. Here, we show that promoters of human YY1 (Yin and Yang 1) as well as caspase 7 genes are modulated by prohibitin. YY1 promoter activity was reduced upon overexpression of prohibitin, while it was enhanced when prohibitin was depleted by small interfering RNA techniques. The repressive effects of prohibitin on the YY1 promoter were mediated through E2F binding sites, as seen by mutational analysis and chromatin immunoprecipitation assays. Further, depletion of E2F1 prevented prohibitin from repressing the YY1 promoter. In contrast with YY1, prohibitin overexpression led to enhanced levels of caspase 7, whereas depletion of prohibitin reduced it. Interestingly, the caspase 7 promoter was found to have p53-binding sites and prohibitin activated this promoter through p53. These studies show that prohibitin can have diverse effects on the expression of different genes and the activity of various cellular promoters is affected by prohibitin. Further, it appears very likely that prohibitin carries out many of its cellular functions by affecting the transcription of different genes.

Caspase 7↗

PubMed2005

Camptothecin induces nuclear export of prohibitin preferentially in transformed cells through a CRM-1-dependent mechanism.

Prohibitin is a growth-suppressive protein that has multiple functions in the nucleus and the mitochondria. Our earlier studies had shown that prohibitin represses the activity of E2F transcription factors while enhancing p53-mediated transcription. At the same time, prohibitin has been implicated in mediating the proper folding of mitochondrial proteins. We had found that treatment of cells with camptothecin, a topoisomerase 1 inhibitor, led to the export of prohibitin and p53 from the nucleus to the mitochondria. Here we show that the camptothecin-induced export of prohibitin occurs preferentially in transformed cell lines, but not in untransformed or primary cells. Cells that did not display the translocation of prohibitin were refractive to the apoptotic effects of camptothecin. The translocation was mediated by a putative nuclear export signal at the C-terminal region of prohibitin; fusion of the nuclear export signal (NES) of prohibitin to green fluorescence protein led to its export from the nucleus. Leptomycin B could inhibit the nuclear export of prohibitin showing that it was a CRM-1-dependent event driven by Ran GTPase. Confirming this, prohibitin was found to physically interact with CRM-1, and this interaction was significantly higher in transformed cells. Delivery of a peptide corresponding to the NES of prohibitin prevented the export of prohibitin to cytoplasm and protected cells from apoptosis. These results suggest that the regulated translocation of prohibitin from the nucleus to the mitochondria facilitates its pleiotropic functions and might contribute to its anti-proliferative and tumor suppressive properties.

Active Transport, Cell Nucleus↗

PubMed2006

Prohibitin is a novel target gene of vitamin D involved in its antiproliferative action in breast cancer cells.

Previously, we showed that N-methyl-N-nitrosourea-transformed MCF12F breast epithelial cells exhibited differential expression of several genes, including up-regulation of prohibitin and elevated sensitivity to a relatively noncalcemic vitamin D analogue, 1alpha-hydroxyvitamin D5 [1alpha(OH)D5]. In this report, we evaluated the functional significance of prohibitin in relation to the cellular response to vitamin D. The in silico screening for putative transcription factor binding sites identified two vitamin D receptor (VDR)/retinoid X receptor binding sites in the 1-kb promoter region of prohibitin. Prohibitin up-regulation by 1alpha(OH)D5 treatment at both transcriptional and translational levels was confirmed by real-time reverse transcription-PCR and Western blot analysis in breast cancer cells, identifying prohibitin as a vitamin D target gene. Confocal microscopic analysis showed that prohibitin was localized in the nuclei of MCF-7 cells and a portion of prohibitin was colocalized with VDR, but direct physical interaction between VDR and prohibitin in cell lysates was not detectable. In MCF-7 cells expressing tetracycline-inducible prohibitin (Tet-On model), the overexpression of prohibitin inhibited cell proliferation and enhanced vitamin D-induced antiproliferative activity. Knockdown of prohibitin was accompanied by increased number of cells incorporating bromodeoxyuridine in the whole population and increased cell distribution in the S phase of cell cycle. In addition, prohibitin level had no significant effect on the vitamin D-induced transactivation of CYP24, a VDR target gene. This is the first report to suggest that prohibitin serves as a novel vitamin D target gene, which is involved in the antiproliferative action of vitamin D without affecting CYP24 transactivation in breast cancer cells.

Base Sequence↗

PubMed1993

Absence of prohibitin gene mutations in human epithelial ovarian tumors.

Multiple loss of heterozygosity (LOH) studies of ovarian cancers have found a high incidence of chromosome 17 loss in these tumors. Several authors have suggested that the region commonly deleted encompasses 17q12-21. In addition, this region has recently been reported to be linked to the familial breast/ovarian cancer syndrome. Recently the human prohibitin gene was mapped to region 17q12-22. Prohibitin causes arrest of DNA synthesis by fibroblast and HeLa cells and prohibitin shows significant homology to a gene (Cc) thought to be important for the regulation of development of Drosophila melanogaster. These findings have led many to consider the prohibitin gene a potential tumor suppressor gene. In addition, sequence analysis of exon 4 of human prohibitin gene revealed mutations in 4 of 23 sporadic breast carcinomas. Because of the proposed function for prohibitin, its alterations in breast cancers, and the fact that its location on 17q falls within a commonly deleted region in ovarian cancers, we have undertaken an analysis of the sequence of prohibitin in epithelial ovarian cancers. Using several polymorphic DNA probes, we identified 20 epithelial ovarian tumors which demonstrated LOH for the region that contains the prohibitin gene. To evaluate whether mutations of prohibitin may be important in ovarian carcinogenesis, we have sequenced exons 4 and 5 of this gene using the technique of genomic amplification with transcript sequencing. Only normal exon 4 and 5 sequence was observed among the 20 tumors screened. These results demonstrate that this region of the prohibitin gene is not mutated in epithelial ovarian cancers and suggest that the prohibitin gene does not play a role in ovarian carcinogenesis. Sequencing of further exons and introns are needed to confirm this latter hypothesis.

Base Sequence↗

PubMed2002

Prohibitin co-localizes with Rb in the nucleus and recruits N-CoR and HDAC1 for transcriptional repression.

The potential tumor suppressor protein prohibitin can prevent cell proliferation and this required its binding to the Rb protein. Prohibitin could repress the transcriptional activity of E2F family members and this required a part of the marked box region of E2F. The sub-cellular localization of prohibitin has been variously attributed to the mitochondria as well as the inner cell membrane. Here we show that a subset of prohibitin molecules are present in the nucleus where it co-localizes with the Rb protein. Deletion of a putative amino-terminal membrane-docking domain of prohibitin had no effect on its ability to suppress cell proliferation or inhibit E2F activity. Our experiments show that a 53 amino-acid stretch of E2F1 is sufficient for being targeted by prohibitin; fusion of this region to GAL4-VP16 construct could make it susceptible to prohibitin-mediated, but not Rb-mediated repression. Prohibitin, like Rb, could repress transcription from SV40 and major late promoters when recruited directly to DNA. Prohibitin mediated transcriptional repression required histone-deacetylase activity, but unlike Rb, additional co-repressors like N-CoR are also involved. Repression by prohibitin correlates with histone deacetylation on promoters and this was reversed by IgM stimulation of cells; IgM did not affect Rb-mediated repression or deacetylation of the promoters. Prohibitin thus appears to repress E2F-mediated transcription utilizing different molecular mediators and facilitate channeling of specific signaling pathways to the cell cycle machinery.

Base Sequence↗

PubMed2004

Regulation of prohibitin expression during follicular development and atresia in the mammalian ovary.

Prohibitin is a ubiquitous and highly conserved protein implicated as an important regulator in cell survival. Prohibitin content is inversely associated with cell proliferation, but it increases during granulosa cell differentiation as well as in earlier events of apoptosis in a temperature-sensitive granulosa cell line. In the present study, we have characterized the spatial expression patterns for prohibitin using established in vivo models for the induction of follicular development and atresia in the mammalian ovary. Comparative Western blot analyses of granulosa cell lysates from control ovaries and from ovaries primed with eCG or treated with eCG plus anti-eCG (gonadotropin withdrawal) were conducted. Prohibitin was immunolocalized in rat ovarian sections probed with antibodies against either proliferating cell nuclear antigen (PCNA) or cholesterol side-chain cleavage cytochrome P450 (P450(scc)) or in terminal deoxynucleotidyl transferase-mediated dUTP nick end labeled sections. Additionally, porcine oocytes, zygotes, and blastocyts were also immunolocalized with prohibitin antibody. Immunolocalization revealed the presence of prohibitin in granulosa cells, theca-interstitial cells, and the oocyte. The results indicate that prohibitin protein expression in the gonadotropin-treated cells was upregulated. Immunoreactivity of prohibitin was inversely related to PCNA expression during follicular maturation and colocalized with P450(scc). Prohibitin appeared to be translocated from the cytoplasm to the nucleus in atretic follicles, germinal vesicle-stage oocytes, zygotes, and blastocysts. These results suggest that prohibitin has several functional regulatory roles in granulosa and theca-interstitial cells and in the ovum during follicular maturation and atresia. It is likely that prohibitin may play an important role in determining the fate of these cells and eventual follicular destiny.

Animals↗

PubMed2006

Prohibitin facilitates cellular senescence by recruiting specific corepressors to inhibit E2F target genes.

Prohibitin is a growth regulatory gene that has pleiotropic functions in the nucleus, mitochondria, and cytoplasmic compartments. Earlier studies had proposed a role for prohibitin in modulating cellular senescence, but the underlying mechanisms remain unknown. Here we show that senescence induced by DNA-damaging agents causes the localization of prohibitin to specific heterochromatic foci. Prohibitin could bind to heterochromatin protein 1 (HP1) family proteins and colocalized with HP1gamma in senescence-associated heterochromatic foci. Further, HP1gamma could synergize with prohibitin to repress E2F1-mediated transcriptional activity. The depletion of prohibitin by small interfering RNA or antisense techniques led to a reduction in the senescent phenotype, correlating with a reduced expression of senescence-associated beta-galactosidase and fewer numbers of senescence-associated heterochromatic foci. Chromatin immunoprecipitation assays showed that prohibitin is needed for the recruitment of HP1gamma to E2F1-regulated proliferative promoters, leading to their repression. The ablation of prohibitin prevented the recruitment of HPIgamma, but not Suv39H, to the promoters upon senescence. Prohibitin-mediated recruitment of HP1gamma occurred in only senescent cells, not in quiescent cells; thus, there is a dichotomy in the recruitment of different corepressors by prohibitin, depending on the type of growth arrest. These studies show that prohibitin plays a vital role in inducing cellular senescence.

Cell Line, Tumor↗

PubMed1999

Immunolocalization and expression of prohibitin, a mitochondrial associated protein within the rat ovaries.

This study was designed to determine the cellular distribution and pattern of expression for the mitochondria-associated protein, prohibitin, during the transitional stages of follicular differentiation within the rat ovary. Immunohistochemical staining techniques were used on frozen sections to examine the localization of prohibitin to preantral, antral, preovulatory, and atretic follicles. Prohibitin localization was also determined in corpus luteum from adult rats, in addition to those from infant and juvenile ovaries, before and after gonadotropin stimulation. Western and Northern blotting techniques were used for qualitative and quantitative assessment of prohibitin expression levels within the ovary. Prohibitin was localized within granulosa cells of infant and juvenile ovaries in a relatively heterogeneous staining pattern. The oocyte also exhibited robust prohibitin expression at all stages of follicular development. In addition, strong prohibitin expression was evident in the corpus luteum as well as in follicles undergoing atresia. Additional data derived from studies involving a GnRH-agonist indicate that increases in prohibitin protein expression correlate with the initial events of apoptosis. Collectively, these results support a growth regulatory role for prohibitin within the rat ovary. Therefore, we propose that prohibitin may serve as an important regulator of granulosa cell fate during follicular development.

Animals↗

PubMed2003

Prohibitin induces the transcriptional activity of p53 and is exported from the nucleus upon apoptotic signaling.

Prohibitin, a potential tumor suppressor protein, has been shown to inhibit cell proliferation and repress E2F transcriptional activity. Though prohibitin has potent transcriptional functions in the nucleus, a mitochondrial role for prohibitin has also been proposed. Here we show that prohibitin is predominantly nuclear in two breast cancer cell lines where it co-localizes with E2F1 and p53. Upon apoptotic stimulation by camptothecin, prohibitin is exported to perinuclear regions where it localizes to mitochondria. The data presented here also show that prohibitin is capable of physically interacting with p53 in vivo and in vitro. Prohibitin was found to enhance p53-mediated transcriptional activity and cotransfection of an antisense prohibitin construct reduces p53-mediated transcriptional activation. Prohibitin appears to induce p53-mediated transcription by enhancing its recruitment to promoters, as detected by chromatin immunoprecipitation assays. These results suggest that prohibitin is capable of modulating Rb/E2F as well as p53 regulatory pathways.

Active Transport, Cell Nucleus↗

PubMed2002

Prohibitin requires Brg-1 and Brm for the repression of E2F and cell growth.

E2F transcription factors play a major role in controlling mammalian cell cycle progression. We recently reported that a potential tumor suppressor, prohibitin, which interacts with retinoblastoma protein (Rb), regulates E2F function and this activity correlates with its growth-suppressive activity. We show here that prohibitin recruits Brg-1/Brm to E2F-responsive promoters, and that this recruitment is required for the repression of E2F-mediated transcription by prohibitin. Expression of a dominant-negative Brg-1 or Brm releases prohibitin-mediated repression of E2F and relieves prohibitin-mediated growth suppression. Although prohibitin associates with, and recruits, Brg-1 and Brm independently of Rb, prohibitin/Brg-1/Brm-mediated transcriptional repression requires Rb. A viral oncoprotein, SV40 large T antigen, can reverse prohibitin-mediated suppression of E2F-mediated gene transcription, and targets prohibitin through interruption of the association between prohibitin and Brg-1/Brm without affecting the prohibitin-E2F interaction.

Antigens, Polyomavirus Transforming↗

PubMed1999

Rb and prohibitin target distinct regions of E2F1 for repression and respond to different upstream signals.

E2F transcription factor is subject to stringent regulation by a variety of molecules. We recently observed that prohibitin, a potential tumor suppressor protein, binds to the retinoblastoma (Rb) protein and represses E2F transcriptional activity. Here we demonstrate that prohibitin requires the marked box region of E2F for repression; further, prohibitin can effectively inhibit colony formation induced by overexpression of E2F1 in T47D cells. Prohibitin was also found to interact with the signaling kinase c-Raf-1, and Raf-1 could effectively reverse prohibitin-mediated repression of E2F activity. Agents such as E1A, p38 kinase, and cyclins D and E had no effect on prohibitin-mediated repression of E2F1, but all of these molecules could reverse Rb function. Similarly, stimulation of the immunoglobulin M signaling pathway in Ramos cells could inactivate prohibitin, but this had no effect on Rb function. Serum stimulation of quiescent Ramos cells inactivated Rb and prohibitin with different kinetics; further, while the serum-dependent inactivation of Rb was dependent on cyclin-dependent kinase activity, the inactivation of prohibitin was not. We believe that prohibitin is a novel regulator of E2F function which channels specific signaling cascades to the cell cycle regulatory machinery.

Binding Sites↗

PubMed2006

Apoptosis of rat granulosa cells after staurosporine and serum withdrawal is suppressed by adenovirus-directed overexpression of prohibitin.

Prohibitin (Phb1) is a highly conserved mitochondrial protein that is associated with granulosa cell differentiation, atresia, and luteolysis. Although prohibitin has been implicated in the suppression of apoptosis in mammalian cells, its specific role in programmed cell death in granulosa cells is unknown. In the present study, we examined the role of prohibitin in mediating staurosporine (STS) and serum withdrawal induced apoptosis in undifferentiated rat granulosa cells. Treatment of granulosa cells isolated from immature rat ovaries with STS and/or serum withdrawal induced a rapid decrease in the transmembrane potential of mitochondria, resulting in increased prohibitin content and induced apoptosis in a time- and dose-dependent manner. Infection of granulosa cells with a Phb1 adenoviral construct resulted in overexpression of prohibitin that markedly attenuated the ability of STS and serum withdrawal to induce apoptosis via the intrinsic apoptotic pathway. To determine the site of action of Phb1, granulosa cells were transfected with a prohibitin-eGFP fusion construct, and the fusion protein expression patterns were analyzed by fluorescence microscopy and Western blot analysis of cell fractionated samples. These studies indicated that the prohibitin-eGFP fusion protein moved from the cytoplasm into the mitochondria. However, no prohibitin-eGFP fusion protein was observed in the nucleus in response to the STS-induced apoptotic stimulus. This result was corroborated by Western blot analysis with green fluorescent protein-specific antibody. Furthermore, the prohibitin-eGFP fusion protein also inhibited programmed cell death. These results provide evidence that prohibitin could serve an antiapoptotic role in undifferentiated granulosa cells.

Adenoviridae↗

PubMed1996

Prohibitin in breast cancer cell lines: loss of antiproliferative activity is linked to 3' untranslated region mutations.

The evolutionarily conserved prohibitin gene is located on human chromosome 17q21, and two alleles have been identified. Our previous studies characterizing prohibitin in immortalized cells, classified into four complementation groups (A-D) based on the ability of whole-cell hybrids to become senescent, have suggested that it has tumor suppressor activity in group B cells. Only the cell lines assigned to group B are sensitive to the antiproliferative activity of prohibitin, and all are homozygous for an allele designated B because of its exclusive association with this group. Prohibitin genotyping of 22 breast cancer cell lines identified 17 homozygous for the B allele, 5 homozygous for the non-B allele, and no heterozygotes. Four of these cell lines were chosen for further characterization of prohibitin. In cell proliferation assays, the homozygous B breast cancer cell lines (BT-20, SK-BR-3, and MCF7) are all inhibited from traversing the cell cycle following the introduction of wild-type prohibitin transcripts. The cell line homozygous for the alternative non-B allele (BT-549) is not inhibited by transcripts. All of the breast cancer cell lines overexpress the longer form of the prohibitin mRNA (1.9 kb) and the protein. Mutational analysis of the protein-coding region detected no mutations in any of the lines. However, BT-20, SK-BR-3, and MCF7 cells are all mutated in the final 200 bases of the 3' untranslated region (3'UTR) exclusive to the 1.9-kb transcript, but BT-549 cells had no alterations in this region of the 3'UTR. Functional mapping experiments performed in the mutated SK-BR-3 line showed that the wild-type 3'UTR alone is sufficient to inhibit cell cycle progression, indicating that the antiproliferative activity of the prohibitin transcript is localized to this region. Overall, our results show that most (80%) of the cell lines derived from breast tumors have a common prohibitin genotype, suggesting that they belong to the same group of immortalized cells, group B. The results also show that the prohibitin 3'UTR exhibits the characteristics of a trans-acting regulatory RNA (riboregulator), the tumor suppressor activity of which is inactivated by mutation in group B immortalized cells.

Alleles↗

PubMed1996

The 3' untranslated region of prohibitin and cellular immortalization.

We have been studying the role of the evolutionarily conserved prohibitin gene in cellular immortalization and tumor suppression. Immortalized human cells are classified into four complementation groups (A, B, C, and D) based on the ability of fusion hybrids to become senescent. The present study expands our preliminary evidence showing that the antiproliferative activity of prohibitin is only effective in immortalized Group B cells and normal cells. Data presented here show that the expression of a prohibitin mRNA with a long 3' untranslated region (3'UTR) and prohibitin protein is elevated in immortalized cells from all complementation groups. However, all immortalized cells classified in complementation Group B, and no cell lines in any of the other groups, are sensitive to the antiproliferative activity of wild-type prohibitin transcripts. All Group B cells are also homozygous for one of two human prohibitin alleles that are distinguishable by two distinct intron polymorphism restriction sites. Interestingly, sequence analysis of the prohibitin gene from representatives of each of the complementation groups showed that the 3'UTR from Groups A, C, and D matched wild type; however, the sequence from all four Group B cell lines differed from wild type. Functional inhibition assays on truncated wild-type mRNA transcripts as well as 3'UTR specific wild-type and mutated transcripts show that the antiproliferative activity of prohibitin resides, at least in part, in the 3'UTR. These data suggest that the prohibitin 3'UTR may function as a trans-acting regulatory RNA (riboregulator) whose tumor suppressor activity has been inactivated by mutation in Group B cells.

Base Sequence↗

PubMed2001

Characterization of prohibitin in a newly established rat ovarian granulosa cell line.

Prohibitin is an evolutionary conserved protein that is associated with cellular differentiation, atresia, and luteolysis in the rat ovary. However, the specific cellular location and function of prohibitin in ovarian cells has not been clearly elucidated. To characterize the expression of prohibitin during cell proliferation, differentiation, and cell death, we have successfully established a temperature-sensitive granulosa cell line, designated RGA-1. At a permissive temperature of 33 C, RGA-1 cells proliferate, but revert to a differentiated phenotype at a nonpermissive temperature of 39 C. Significant inductions of prohibitin mRNA and protein expression were observed in the differentiated phenotype when compared with proliferating cells. Differentiated RGA-1 cells were found to express inhibin alpha- and beta-transcripts, as well as steroidogenic acute regulatory protein and peripheral-type benzodiazepine receptor proteins in a manner reminiscent of steroidogenic functional responses observed in primary differentiated granulosa cells. Prohibitin expression correlated well with the expression of these steroidogenic proteins. At 39 C, RGA-1 cells also displayed increases in p53 protein levels, indicative of growth arrest in the nonproliferating cells. Confocal and electron microscopic examinations revealed increased prohibitin localization to the mitochondria at 39 C, along with changes in mitochondrial size and shape. These changes were accompanied by marked reductions in cytochrome c oxidase subunit II levels and in unit mitochondrial transmembrane potential. In addition, cell fractionation studies demonstrated that the prohibitin protein was mainly localized to the mitochondrial membrane. Collectively, these findings suggest a role for prohibitin in mitochondrial structure and function during growth and differentiation in ovarian granulosa cells. Prohibitin expression may also be indicative of mitochondrial destabilization during apoptosis-related events.

Animals↗

PubMed2002

The shortened replicative life span of prohibitin mutants of yeast appears to be due to defective mitochondrial segregation in old mother cells.

Prohibitin proteins have been implicated in cell proliferation, aging, respiratory chain assembly and the maintenance of mitochondrial integrity. The prohibitins of Saccharomyces cerevisiae, Phb1 and Phb2, have strong sequence similarity with their human counterparts prohibitin and BAP37, making yeast a good model organism in which to study prohibitin function. Both yeast and mammalian prohibitins form high-molecular-weight complexes (Phb1/2 or prohibitin/BAP37, respectively) in the inner mitochondrial membrane. Expression of prohibitins declines with senescence, both in mammalian fibroblasts and in yeast. With a total loss of prohibitins, the replicative (budding) life span of yeast is reduced, whilst the chronological life span (the survival of stationary cells over time) is relatively unaffected. This effect of prohibitin loss on the replicative life span is still apparent in the absence of an assembled respiratory chain. It also does not reflect the production of extrachromosomal ribosomal DNA circles (ERCs), a genetic instability thought to be a major cause of replicative senescence in yeast. Examination of cells containing a mitochondrially targeted green fluorescent protein indicates this shortened life span is a reflection of defective mitochondrial segregation from the mother to the daughter in the old mother cells of phb mutant strains. Old mother phb mutant cells display highly aberrant mitochondrial morphology and, frequently, a delayed segregation of mitochondria to the daughter. They often arrest growth with their last bud strongly attached and with the mitochondria adjacent to the septum between the mother and the daughter cell.

Cell Division↗

PubMed1997

Prohibitin, a putative negative control element present in Pneumocystis carinii.

Little is known about the molecules involved in the regulation of Pneumocystis carinii replication and development in vitro and in vivo. We describe in this report the identification of a P. carinii gene encoding the P. carinii prohibitin protein. In mammals, the prohibitin gene product has been shown to negatively regulate cell proliferation. A cDNA clone encoding the P. carinii prohibitin gene was isolated from a P. carinii cDNA library and identified on the basis of amino acid sequence homology with prohibitin from mammalian sources. Southern blot analysis confirmed that the prohibitin cDNA clone was of P. carinii origin. Western blot analysis of total P. carinii protein indicated that the prohibitin gene is transcribed and translated in vivo. The P. carinii prohibitin gene was expressed in vivo in human fibroblasts and shown to arrest the cell cycle in the G1 phase. The results obtained suggest a potential role for P. carinii prohibitin in the regulation of P. carinii proliferation and development.

Amino Acid Sequence↗