Establishment of cell polarity in yeast.
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Cell Cycle↗
Biomedical subjects
J Chant
Publications and source records attributed to J Chant.
Multifunctional proteins. Calmodulin clarified.
Genetic analysis in yeast is helping to dissect the multiple functions of calmodulin: mutations have been made that uncouple calmodulin from single targets among many.
Calmodulin↗
Cell polarity in yeast.
Cell polarity is fundamental to the development and functioning of all organisms, from bacteria to humans. Examples of processes that involve cell polarity include the growth of axons, the interaction between T cells and their targets, the formation of buds by yeast, and sporulation in Bacillus spp. Recent work on budding yeast has provided valuable insights into the molecular machinery responsible for establishing and orienting cell polarity. Comparisons of the DNA sequences of genes involved in such pathways have raised the possibility that these mechanisms are conserved in all eukaryotic cells.
Animals↗
BUD2 encodes a GTPase-activating protein for Bud1/Rsr1 necessary for proper bud-site selection in yeast.
Cells of the budding yeast Saccharomyces cerevisiae bud at either axial or bipolar sites depending on their cell type. Bud-site selection directs both cell polarity and the cytoskeletal orientation during budding and is determined by at least five genes: BUD1/RSR1, BUD2, BUD3, BUD4 and BUD5. Mutants defective in BUD1, BUD2 or BUD5 choose bud sites randomly. Bud1 protein (Bud1p) has sequence similarity to Ras, a small GTP-binding protein, and Bud5p is similar to Cdc25p (refs 4, 5), a GDP-GTP exchange factor. Here we report that Bud2p is a GTPase-activating protein (GAP) for Bud1p with a sequence similar to the catalytic domain of rasGAPs, and that Bud2p purified from yeast stimulates GTP hydrolysis by Bud1p. Chromosomal deletion of BUD2 causes a random budding pattern but no obvious growth defect. Overexpression of BUD2 also causes a random budding pattern by wild-type cells.
Amino Acid Sequence↗
Genetic control of bud site selection in yeast by a set of gene products that constitute a morphogenetic pathway.
Yeast cells choose bud sites on their surface in two distinct spatial patterns: axial for a and alpha cells and bipolar for a/alpha cells. We have identified four genes, BUD1-BUD4, necessary for the axial pattern by isolating mutants of alpha cells that do not exhibit this pattern. Mutations in BUD1 (which is the same as the previously identified gene RSR1) or BUD2 lead to a random budding pattern in all cell types; mutations in BUD3 or BUD4 lead to a bipolar pattern in all cell types. These observations indicate the existence of a basal budding pattern, requiring no BUD products, that is random; BUD1 and BUD2 act on this basal pattern to create the bipolar pattern; the further action of BUD3 and BUD4 leads to the axial pattern. These studies thus identify a set of gene products that directs cell morphogenesis to a genetically programmed site.
Animals↗
Yeast BUD5, encoding a putative GDP-GTP exchange factor, is necessary for bud site selection and interacts with bud formation gene BEM1.
Cells of the yeast S. cerevisiae choose bud sites in an axial or bipolar spatial pattern depending on their cell type. We have identified a gene, BUD5, that resembles BUD1 and BUD2 in being required for both patterns; bud5- mutants also exhibit random budding in all cell types. The BUD5 nucleotide sequence predicts a protein of 538 amino acids that has similarity to the S. cerevisiae CDC25 product, an activator of RAS proteins that catalyzes GDP-GTP exchange. Two potential targets of BUD5 are known: BUD1 (RSR1) and CDC42, proteins involved in bud site selection and bud formation, respectively, that have extensive similarity to RAS. We also show that BUD5 interacts functionally with a gene, BEM1, that is required for bud formation. This interaction provides further support for the view that products involved in bud site selection guide the positioning of a complex necessary for bud formation.
Amino Acid Sequence↗
Budding and cell polarity in Saccharomyces cerevisiae.
Budding by yeast follows a sequence of three stages. These include selection of a non-random bud-site, organization of that site and establishment of an associated axis of cytoskeletal polarity, and localized growth of the cell surface to produce the bud. Numerous components involved in each stage have been identified. As some of these components have close homologs in other organisms, there may exist common mechanisms involved in the establishment of cell polarity.
Animal Population Groups↗
Archaebacteria: transcription and processing of ribosomal RNA sequences in Halobacterium cutirubrum.
The chromosome of Halobacterium cutirubrum contains a single ribosomal RNA gene cluster. The 5' to 3' organization of genes within this 6-kpb region is: 16S, alanine tRNA, 23S, 5S, cysteine tRNA. The entire gene cluster is transcribed as a single long primary transcript; processing of mature RNA sequences from the 5' region of the transcript begins prior to the completion of synthesis at the 3' end. There are five conserved octanucleotide direct repeats (TGCGAACG) in the 900-bp 5'-flanking sequence in front of the 16S gene. The positions of these repeat sequences correspond to the different 5' ends of the primary transcript and probably represent the RNA polymerase start sites. The 16S and 23S rRNA genes are surrounded by long nearly perfect inverted repeat sequences. These sequences probably form duplex structures in the primary transcript and are recognized by an RNaseIII-like endonuclease activity that carries out the initial excision of the precursor 16S and 23S rRNA sequences. These precursors are rapidly trimmed tot he mature 16S and 23S molecules and assembled into ribosomal particles. The processing sites for 5S rRNA appear to be at or very near to the mature ends of the 5S molecule. The tRNA sequences are processed with reduced efficiency from the primary transcript. Nuclease cuts have been detected at the ends as well as in the middle of the cysteine tRNA sequence suggesting that there may be alternative processing pathways, one resulting in proper excision of the mature tRNA sequence and the other resulting in improper excision and degradation of the tRNA sequence. The transcription termination sequence is believed to be at or beyond an AT-rich sequence preceded by a GC-rich sequence located distal to the cysteine tRNA gene.
Base Sequence↗
Cyk3, a novel SH3-domain protein, affects cytokinesis in yeast.
Cytokinesis requires the wholesale reorganization of the cytoskeleton and secretion to complete the division of one cell into two. In the budding yeast Saccharomyces cerevisiae, the IQGAP-related protein Iqg1 (Cyk1) promotes cytokinetic actin ring formation and is required for cytokinesis and viability [1-3]. As the actin ring is not essential for cytokinesis or viability, Iqg1 must act by another mechanism [4]. To uncover this mechanism, a screen for high-copy suppressors of the iqg1 lethal phenotype was performed. CYK3 suppressed the requirement for IQG1 in viability and cytokinesis without restoration of the actin ring, demonstrating that CYK3 promotes cytokinesis through an actomyosin-ring-independent pathway. CYK3 encodes a novel SH3-domain protein that was found in association with the actin ring and the mother-bud neck. cyk3 null cells had misshapen mother-bud necks and were deficient in cytokinesis. In the cyk3 null strain, actin rearrangements associated with cytokinesis appeared normal, suggesting that the phenotype reflects a defect in secretory targeting or septal synthesis. Deletion of either cyk3 or hof1 alone results in a mild cytokinetic phenotype [5-7], but deletion of both genes resulted in lethality and a complete cytokinetic block, suggesting overlapping function. Thus, Cyk3 appears to be important for cytokinesis and acts potentially downstream of Iqg1.
Amino Acid Sequence↗