Patterns of bud-site selection in the yeast Saccharomyces cerevisiae

Rolled-up transparent microtubes as two-dimensionally confined culture scaffolds of individual yeast cells

Transparent oxide rolled-up microtube arrays were constructed on Si substrates by the deposition of a pre-stressed oxide layer on a patterned photoresist sacrificial layer and the subsequent removal of this sacrificial layer. These microtubes as well as their arrays can be well positioned onto a chip for further applications, while their dimensions (e.g. length, diameter and wall thickness) are controlled by tunable parameters of the fabrication process. Due to the unique tubular structure and optical transparency, such rolled-up microtubes can serve as well-defined two-dimensionally (2D) confined cell culture scaffolds. In our experiments, yeast cells exhibit different growth behaviors (i.e. their arrangement) in microtubes with varied diameters. In an extremely small microtube the yeast cell becomes highly elongated during growth but still survives. Detailed investigations on the behavior of individual yeast cells in a single microtube are carried out in situ to elucidate the mechanical interaction between microtubes and the 2D confined cells. The confinement of tubular channels causes the rotation of cell pairs, which is more pronounced in smaller microtubes, leading to different cellular assemblies. Our work demonstrates good capability of rolled-up microtubes for manipulating individual and definite cells, which promises high potential in lab-on-a-chip applications, for example as a bio-analytic system for individual cells if integrated with sensor functionalities.

Of bars and rings: Hof1-dependent cytokinesis in multiseptated hyphae of Ashbya gossypii

We analyzed the development of multiple septa in elongated multinucleated cells (hyphae) of the filamentous ascomycete Ashbya gossypii in which septation is apparently uncoupled from nuclear cycles. A key player for this compartmentalization is the PCH protein Hof1. Hyphae that are lacking this protein form neither actin rings nor septa but still elongate at wild-type speed. Using in vivo fluorescence microscopy, we present for the first time the coordination of cytokinesis and septation in multiseptated and multinucleated cells. Hof1, the type II myosin Myo1, the landmark protein Bud3, and the IQGAP Cyk1 form collars of cortical bars already adjacent to hyphal tips, thereby marking the sites of septation. While hyphae continue to elongate, these proteins gradually form cortical rings. This bar-to-ring transition depends on Hof1 and Cyk1 but not Myo1 and is required for actin ring assembly. The Fes/CIP4 homology (FCH) domain of Hof1 ensures efficient localization of Hof1, whereas ring integrity is conferred by the Src homology 3 (SH3) domain. Up to several hours after site selection, actin ring contraction leads to membrane invagination and subsequent cytokinesis. Simultaneously, a septum forms between the adjacent hyphal compartments, which do not separate. During evolution, A. gossypii lost the homologs of two enzymes essential for cell separation in Saccharomyces cerevisiae.

Spore germination in Saccharomyces cerevisiae: global gene expression patterns and cell cycle landmarks

Genome-wide expression profiling of spore germination in Saccharomyces cerevisiae reveals two major stages and identifies germination-specific regulation of cell cycle machinery.

Background

Spore germination in the yeast Saccharomyces cerevisiae is a process in which non-dividing haploid spores re-enter the mitotic cell cycle and resume vegetative growth. To study the signals and pathways underlying spore germination we examined the global changes in gene expression and followed cell-cycle and germination markers during this process.

Results

We find that the germination process can be divided into two distinct stages. During the first stage, the induced spores respond only to glucose. The transcription program during this stage recapitulates the general transcription response of yeast cells to glucose. Only during the second phase are the cells able to sense and respond to other nutritional components in the environment. Components of the mitotic machinery are involved in spore germination but in a distinct pattern. In contrast to the mitotic cell cycle, growth-related events during germination are not coordinated with nuclear events and are separately regulated. Thus, genes that are co-induced during G1/S of the mitotic cell cycle, the dynamics of the septin Cdc10 and the kinetics of accumulation of the cyclin Clb2 all exhibit distinct patterns of regulation during spore germination, which allow the separation of cell growth from nuclear events.

Conclusion

Taken together, genome-wide expression profiling enables us to follow the progression of spore germination, thus dividing this process into two major stages, and to identify germination-specific regulation of components of the mitotic cell cycle machinery.

Cleavage of the signaling mucin Msb2 by the aspartyl protease Yps1 is required for MAPK activation in yeast

Signaling mucins are cell adhesion molecules that activate RAS/RHO guanosine triphosphatases and their effector mitogen-activated protein kinase (MAPK) pathways. We found that the Saccharomyces cerevisiae mucin Msb2p, which functions at the head of the Cdc42p-dependent MAPK pathway that controls filamentous growth, is processed into secreted and cell-associated forms. Cleavage of the extracellular inhibitory domain of Msb2p by the aspartyl protease Yps1p generated the active form of the protein by a mechanism incorporating cellular nutritional status. Activated Msb2p functioned through the tetraspan protein Sho1p to induce MAPK activation as well as cell polarization, which involved the Cdc42p guanine nucleotide exchange factor Cdc24p. We postulate that cleavage-dependent activation is a general feature of signaling mucins, which brings to light a novel regulatory aspect of this class of signaling adhesion molecule.

Polarized growth in the absence of F-actin in Saccharomyces cerevisiae exiting quiescence

BACKGROUND

Polarity establishment and maintenance are crucial for morphogenesis and development. In budding yeast, these two intricate processes involve the superposition of regulatory loops between polarity landmarks, RHO GTPases, actin-mediated vesicles transport and endocytosis. Deciphering the chronology and the significance of each molecular step of polarized growth is therefore very challenging.

PRINCIPAL FINDINGS

We have taken advantage of the fact that yeast quiescent cells display actin bodies, a non polarized actin structure, to evaluate the role of F-actin in bud emergence. Here we show that upon exit from quiescence, actin cables are not required for the first steps of polarized growth. We further show that polarized growth can occur in the absence of actin patch-mediated endocytosis. We finally establish, using latrunculin-A, that the first steps of polarized growth do not require any F-actin containing structures. Yet, these structures are required for the formation of a bona fide daughter cell and cell cycle completion. We propose that upon exit from quiescence in the absence of F-actin, secretory vesicles randomly reach the plasma membrane but preferentially dock and fuse where polarity cues are localized, this being sufficient to trigger polarized growth.

Afi1p functions as an Arf3p polarization-specific docking factor for development of polarity

ADP-ribosylation factors (Arfs) are highly conserved small GTPases and are critical components of vesicle trafficking. Yeast Arf3p, despite its similarity to mammalian Arf6, is not required for endocytosis but is involved in polarity development. In this study, we identified an Arf3p interacting protein 1 (Afi1p), which, through its N-terminal conserved region, specifically interacts with GTP-bound Arf3p. Afi1p is distributed asymmetrically at the plasma membrane and is required for polarized distribution of Arf3p but not of an Arf3p guanine nucleotide-exchange factor, Yel1p. However, Afi1p is not required for targeting of Arf3p or Yel1p to the plasma membrane. Like arf3 mutant yeast, afi1 mutant yeast exhibited an abnormal budding pattern and partially delayed actin patch polarization. An Afi1p, (38)KLGP4A-Afi1p, mutated at the Arf3p-binding region, loses its ability to interact with Arf3p and maintain the polarized distribution of Arf3p. Although (38)KLGP4A-Afi1p still possessed a proper polarized distribution, it lost its ability to rescue actin patch polarization in afi1 mutant cells. Our findings demonstrate that Afi1p functions as an Arf3p polarization-specific adapter and participates in development of polarity.

Mutations in the RAM network confer resistance to the thiol oxidant 4,4'-dipyridyl disulfide

Thiol oxidants are expected to have multiple effects in living cells. Hence, mutations giving resistance to such agents are likely to reveal important targets and/or mechanisms influencing the cellular capacity to withstand thiol oxidation. A screen for mutants resistant to the thiol-specific oxidant dipyridyl disulfide (DPS) yielded tao3-516, which is impaired in the function of the RAM signaling network protein Tao3/Pag1p. We suggest that the DPS-resistance of the tao3-516 mutant might be due to deficient cell-cycle-regulated production of the chitinase Cts1p, which functions in post-mitotic cell separation and depends on Tao3p and the RAM network for regulated expression. Consistent with this, deletion of other RAM genes or CTS1 also resulted in increased resistance to DPS. Exposure to DPS caused extensive depolarization of the actin cytoskeleton. We found that tao3-516 is resistant to latrunculin, a specific inhibitor of actin polymerization, and that ram, Deltaace2, and Deltacts1 mutants are resistant to benomyl, a microtubule-destabilizing drug. Since septum build-up depends on the organization of cytoskeletal proteins, the resistance to cytoskeletal stress of Cts1p-deficient mutants might relate to bypass for abnormal septum-associated protein sorting. The broad resistance toward oxidants (DPS, diamide and H(2)O(2)) of the Deltacts1 strain links cell wall function to the resistance to oxidative stress and suggests the existence of targets that are common for these oxidants.

Oxidant resistance in a yeast mutant deficient in the Sit4 phosphatase

Resistance to thiol oxidation can arise from mutations altering redox homeostasis. A Saccharomyces cerevisiae sit4-110 mutant is here described, which was isolated as resistant to the thiol-specific oxidant dipyridyl disulfide (DPS) and which contains a single-residue substitution in the SIT4 gene. Sit4p is a protein phosphatase with multiple roles in signal transduction through the target-of-rapamycin (TOR) pathway. We found that sit4-110 elevates the levels of glutathione. However, this cannot be the (only) cause for the DPS-resistance, since sit4-110 also conferred DPS/H2O2-resistance in a glutathione-deficient strain. Of the known Sit4p substrates, only Tip41p is involved in DPS-resistance; both Delta tip41 deletion and overexpression of the Tip41p target Tap42p resulted in increased DPS-resistance. Thus, the role of Sit4p in DPS-tolerance differs from its role during TOR-inactivation and salt stress. In view of Tap42p's known involvement in actin homeostasis, sit4-110 could compensate for putative actin-related defects caused by DPS. However, sit4-110 has pronounced actin polarization defects under both absence and presence of DPS. A relation between actin homeostasis and DPS resistance of sit4-110 cannot be ruled out, but our results suggest that unknown pathways might be involved in DPS resistance through mechanisms involving the Sit4p and/or Tap42p function(s).

Lipid remodeling of GPI-anchored proteins and its function

Many proteins are attached to the cell surface via a conserved post-translational modification, the glycosylphosphatidylinositol (GPI) anchor. GPI-anchored proteins are functionally diverse, but one of their most striking features is their association with lipid microdomains, which consist mainly of sphingolipids and sterols. GPI-anchored proteins modulate various biological functions when they are incorporated into these specialized domains. The biosynthesis of GPI and its attachment to proteins occurs in the endoplasmic reticulum. The lipid moieties of GPI-anchored proteins are further modified during their transport to the cell surface, and these remodeling processes are essential for the association of proteins with lipid microdomains. Recently, several genes required for GPI lipid remodeling have been identified in yeast and mammalian cells. In this review, we describe the pathways for lipid remodeling of GPI-anchored proteins in yeast and mammalian cells, and discuss how lipid remodeling affects the association of GPI-anchored proteins with microdomains in cellular events.

Adjacent positioning of cellular structures enabled by a Cdc42 GTPase-activating protein-mediated zone of inhibition

Cells of the budding yeast Saccharomyces cerevisiae are born carrying localized transmembrane landmark proteins that guide the subsequent establishment of a polarity axis and hence polarized growth to form a bud in the next cell cycle. In haploid cells, the relevant landmark proteins are concentrated at the site of the preceding cell division, to which they recruit Cdc24, the guanine nucleotide exchange factor for the conserved polarity regulator Cdc42. However, instead of polarizing at the division site, the new polarity axis is directed next to but not overlapping that site. Here, we show that the Cdc42 guanosine triphosphatase-activating protein (GAP) Rga1 establishes an exclusion zone at the division site that blocks subsequent polarization within that site. In the absence of localized Rga1 GAP activity, new buds do in fact form within the old division site. Thus, Cdc42 activators and GAPs establish concentric zones of action such that polarization is directed to occur adjacent to but not within the previous cell division site.

Yeast Rrp14p is a nucleolar protein involved in both ribosome biogenesis and cell polarity

We previously cloned RRP14/YKL082c, whose product exhibits two-hybrid interaction with Ebp2p, a regulatory factor of assembly of 60S ribosomal subunits. Depletion of Rrp14p results in shortage of 60S ribosomal subunits and retardation of processing from 27S pre-rRNA to 25S rRNA. Furthermore, 35S pre-rRNA synthesis appears to decline in Rrp14p-depleted cells. Rrp14p interacts with regulatory factors of 60S subunit assembly and also with Utp11p and Faf1p, which are regulatory factors required for assembly of 40S ribosomal subunits. We propose that Rrp14p is involved in ribosome synthesis from the beginning of 35S pre-rRNA synthesis to assembly of the 60S ribosomal subunit. Disruption of RRP14 causes an extremely slow growth rate of the cell, a severe defect in ribosome synthesis, and a depolarized localization of cortical actin patches throughout the cell cycle. These results suggest that Rrp14p has dual functions in ribosome synthesis and polarized cell growth.

Sequential and distinct roles of the cadherin domain-containing protein Axl2p in cell polarization in yeast cell cycle

Polarization of cell growth along a defined axis is essential for the generation of cell and tissue polarity. In the budding yeast Saccharomyces cerevisiae, Axl2p plays an essential role in polarity-axis determination, or more specifically, axial budding in MATa or alpha cells. Axl2p is a type I membrane glycoprotein containing four cadherin-like motifs in its extracellular domain. However, it is not known when and how Axl2p functions together with other components of the axial landmark, such as Bud3p and Bud4p, to direct axial budding. Here, we show that the recruitment of Axl2p to the bud neck after S/G2 phase of the cell cycle depends on Bud3p and Bud4p. This recruitment is mediated via an interaction between Bud4p and the central region of the Axl2p cytoplasmic tail. This region of Axl2p, together with its N-terminal region and its transmembrane domain, is sufficient for axial budding. In addition, our work demonstrates a previously unappreciated role for Axl2p. Axl2p interacts with Cdc42p and other polarity-establishment proteins, and it regulates septin organization in late G1 independently of its role in polarity-axis determination. Together, these results suggest that Axl2p plays sequential and distinct roles in the regulation of cellular morphogenesis in yeast cell cycle.

Distinct domains of yeast cortical tag proteins Bud8p and Bud9p confer polar localization and functionality

In Saccharomyces cerevisiae, diploid yeast cells follow a bipolar budding program, which depends on the two transmembrane glycoproteins Bud8p and Bud9p that potentially act as cortical tags to mark the cell poles. Here, we have performed systematic structure-function analyses of Bud8p and Bud9p to identify functional domains. We find that polar transport of Bud8p and Bud9p does not depend on N-terminal sequences but instead on sequences in the median part of the proteins and on the C-terminal parts that contain the transmembrane domains. We show that the guanosine diphosphate (GDP)/guanosine triphosphate (GTP) exchange factor Bud5p, which is essential for bud site selection and physically interacts with Bud8p, also interacts with Bud9p. Regions of Bud8p and Bud9p predicted to reside in the extracellular space are likely to confer interaction with the N-terminal region of Bud5p, implicating indirect interactions between the cortical tags and the GDP/GTP exchange factor. Finally, we have identified regions of Bud8p and Bud9p that are required for interaction with the cortical tag protein Rax1p. In summary, our study suggests that Bud8p and Bud9p carry distinct domains for delivery of the proteins to the cell poles, for interaction with the general budding machinery and for association with other cortical tag proteins.

Structural and membrane binding analysis of the Phox homology domain of Bem1p: basis of phosphatidylinositol 4-phosphate specificity

Phox homology (PX) domains, which have been identified in a variety of proteins involved in cell signaling and membrane trafficking, have been shown to interact with phosphoinositides (PIs) with different affinities and specificities. To elucidate the structural origin of the diverse PI specificity of PX domains, we determined the crystal structure of the PX domain from Bem1p that has been reported to bind phosphatidylinositol 4-phosphate (PtdIns(4)P). We also measured the membrane binding properties of the PX domain and its mutants by surface plasmon resonance and monolayer techniques and calculated the electrostatic potentials for the PX domain in the absence and presence of bound PtdIns(4)P. The Bem1p PX domain contains a signature PI-binding site optimized for PtdIns(4)P binding and also harbors basic and hydrophobic residues on the membrane-binding surface. The membrane binding of the Bem1p PX domain is initiated by nonspecific electrostatic interactions between the cationic membrane-binding surface of the domain and anionic membrane surfaces, followed by the membrane penetration of hydrophobic residues. Unlike other PX domains, the Bem1p PX domain has high intrinsic membrane penetrating activity in the absence of PtdIns(4)P, suggesting that the partial membrane penetration may occur before specific PtdIns(4)P binding and last after the removal of PtdIns(4)P under certain conditions. This structural and functional study of the PtdIns(4)P-binding Bem1p PX domain provides new insight into the diverse PI specificities and membrane-binding mechanisms of PX domains.

Central roles of small GTPases in the development of cell polarity in yeast and beyond

SUMMARY

The establishment of cell polarity is critical for the development of many organisms and for the function of many cell types. A large number of studies of diverse organisms from yeast to humans indicate that the conserved, small-molecular-weight GTPases function as key signaling proteins involved in cell polarization. The budding yeast Saccharomyces cerevisiae is a particularly attractive model because it displays pronounced cell polarity in response to intracellular and extracellular cues. Cells of S. cerevisiae undergo polarized growth during various phases of their life cycle, such as during vegetative growth, mating between haploid cells of opposite mating types, and filamentous growth upon deprivation of nutrition such as nitrogen. Substantial progress has been made in deciphering the molecular basis of cell polarity in budding yeast. In particular, it becomes increasingly clear how small GTPases regulate polarized cytoskeletal organization, cell wall assembly, and exocytosis at the molecular level and how these GTPases are regulated. In this review, we discuss the key signaling pathways that regulate cell polarization during the mitotic cell cycle and during mating.

Effects of low-shear modeled microgravity on cell function, gene expression, and phenotype in Saccharomyces cerevisiae

Only limited information is available concerning the effects of low-shear modeled microgravity (LSMMG) on cell function and morphology. We examined the behavior of Saccharomyces cerevisiae grown in a high-aspect-ratio vessel, which simulates the low-shear and microgravity conditions encountered in spaceflight. With the exception of a shortened lag phase (90 min less than controls; P < 0.05), yeast cells grown under LSMMG conditions did not differ in growth rate, size, shape, or viability from the controls but did differ in the establishment of polarity as exhibited by aberrant (random) budding compared to the usual bipolar pattern of controls. The aberrant budding was accompanied by an increased tendency of cells to clump, as indicated by aggregates containing five or more cells. We also found significant changes (greater than or equal to twofold) in the expression of genes associated with the establishment of polarity (BUD5), bipolar budding (RAX1, RAX2, and BUD25), and cell separation (DSE1, DSE2, and EGT2). Thus, low-shear environments may significantly alter yeast gene expression and phenotype as well as evolutionary conserved cellular functions such as polarization. The results provide a paradigm for understanding polarity-dependent cell responses to microgravity ranging from pathogenesis in fungi to the immune response in mammals.

Life within a community: benefit to yeast long-term survival

Traditionally, living organisms have often been classified into two main categories: unicellular and multicellular. In recent years, however, the boundary between these two groups has become less strict and clear than was previously presumed. Studies on the communities formed by unicellular microorganisms have revealed that various properties and processes so far mainly associated with metazoa are also important for the proper development, survival and behaviour of muticellular microbial populations. In this review, we present various examples of this, using a yeast colony as representative of a structured organized microbial community. Among other things, we will show how the differentiation of yeast cells within a colony can be important for the long-term survival of a community under conditions of nutrient shortage, how colony development and physiology can be influenced by the environment, and how a group of colonies can synchronize their developmental changes. In the last section, we introduce examples of molecular mechanisms that can participate in some aspects of the behaviour of yeast populations.

Role of cell cycle-regulated expression in the localized incorporation of cell wall proteins in yeast

The yeast cell wall is an essential organelle that protects the cell from mechanical damage and antimicrobial peptides, participates in cell recognition and adhesion, and is important for the generation and maintenance of normal cell shape. We studied the localization of three covalently bound cell wall proteins in Saccharomyces cerevisiae. Tip1p was found only in mother cells, whereas Cwp2p was incorporated in small-to-medium-sized buds. When the promoter regions of TIP1 and CWP2 (responsible for transcription in early G1 and S/G2 phases, respectively) were exchanged, the localization patterns of Tip1p and Cwp2p were reversed, indicating that the localization of cell wall proteins can be completely determined by the timing of transcription during the cell cycle. The third protein, Cwp1p, was incorporated into the birth scar, where it remained for several generations. However, we could not detect any role of Cwp1p in strengthening the birth scar wall or any functional interaction with the proteins that mark the birth scar pole as a potential future budding site. Promoter-exchange experiments showed that expression in S/G2 phase is necessary but not sufficient for the normal localization of Cwp1p. Studies of mutants in which septum formation is perturbed indicate that the normal asymmetric localization of Cwp1p also depends on the normal timing of septum formation, composition of the septum, or both.

Comparison of cell wall localization among Pir family proteins and functional dissection of the region required for cell wall binding and bud scar recruitment of Pir1p

We examined the localization of the Pir protein family (Pir1 to Pir4), which is covalently linked to the cell wall in an unknown manner. In contrast to the other Pir proteins, a fusion of Pir1p and monomeric red fluorescent protein distributed in clusters in pir1Delta cells throughout the period of cultivation, indicating that Pir1p is localized in bud scars. Further microscopic analysis revealed that Pir1p is expressed inside the chitin rings of the bud scars. Stepwise deletion of the eight units of the repetitive sequence of Pir1p revealed that one unit is enough for the protein to bind bud scars and that the extent of binding of Pir1p to the cell wall depends on the number of these repetitive units. The localization of a chimeric Pir1p in which the repetitive sequence of Pir1p was replaced with that of Pir4p revealed the functional role of the different protein regions, specifically, that the repetitive sequence is required for binding to the cell wall and that the C-terminal sequence is needed for recruitment to bud scars. This is the first report that bud scars contain proteins like Pir1p as internal components.

A Landmark Protein Essential for Establishing and Perpetuating the Polarity of a Bacterial Cell

Polarity is often an intrinsic property of the cell, yet little is known about its origin or its maintenance over generations. Here we identify a landmark protein, TipN, which acts as a spatial and temporal cue for setting up the correct polarity in the bacterium Caulobacter crescentus. TipN marks the new pole throughout most of the cell cycle, and its relocation to the nascent poles at the end of division provides a preexisting reference point for orienting the polarity axis in the progeny. Deletion of tipN causes pleiotropic polarity defects, including frequently reversed asymmetry in progeny size and mislocalization of proteins and organelles. Ectopic localization of TipN along the lateral side of the cell creates new axes of polarity leading to cell branching and formation of competent cell poles. Localization defects of the actin-like protein MreB in the DeltatipN mutant suggest that TipN is upstream of MreB in regulating cell polarity.

The BAR domain proteins: molding membranes in fission, fusion, and phagy

The Bin1/amphiphysin/Rvs167 (BAR) domain proteins are a ubiquitous protein family. Genes encoding members of this family have not yet been found in the genomes of prokaryotes, but within eukaryotes, BAR domain proteins are found universally from unicellular eukaryotes such as yeast through to plants, insects, and vertebrates. BAR domain proteins share an N-terminal BAR domain with a high propensity to adopt alpha-helical structure and engage in coiled-coil interactions with other proteins. BAR domain proteins are implicated in processes as fundamental and diverse as fission of synaptic vesicles, cell polarity, endocytosis, regulation of the actin cytoskeleton, transcriptional repression, cell-cell fusion, signal transduction, apoptosis, secretory vesicle fusion, excitation-contraction coupling, learning and memory, tissue differentiation, ion flux across membranes, and tumor suppression. What has been lacking is a molecular understanding of the role of the BAR domain protein in each process. The three-dimensional structure of the BAR domain has now been determined and valuable insight has been gained in understanding the interactions of BAR domains with membranes. The cellular roles of BAR domain proteins, characterized over the past decade in cells as distinct as yeasts, neurons, and myocytes, can now be understood in terms of a fundamental molecular function of all BAR domain proteins: to sense membrane curvature, to bind GTPases, and to mold a diversity of cellular membranes.

The morphology of Saccharomyces cerevisiae colonies is affected by cell adhesion and the budding pattern

Formation of organized colony morphology is clearly a result of organized, coordinated behavior of cells within a colony, which reflects changes in the cell environment, nutrient availability, inter- and intracolony signaling and others. Under standard conditions, colony morphology is specific to the particular yeast strain, which indicates that reproducibility of the structure appears to be a hallmark of programmed development. Our data indicate that markedly structured morphology of colonies formed by some haploid and diploid Saccharomyces cerevisiae strains is linked to formation of clusters of incompletely separated yeast cells organized into larger aggregates. A prerequisite to aggregate formation appears to be a monopolar budding pattern and the presence of an extracellular matrix and adhesins connecting individual clusters within aggregates. In contrast, less structured colonies of other strains are composed of non-aggregated cells exhibiting different modes of budding (axial/polar/random) in different colony areas. The budding pattern appears to be influenced more by the environment (the presence of surrounding cells, nutrient gradients, etc.) than by strain ploidy. Disruption of the BUD2 gene results in uniform random budding of cells and it partially influences colony morphology.

Forcing interactions as a genetic screen to identify proteins that exert a defined activity

The interaction of proteins to form macromolecular complexes is the basis for most biological processes. Approaches have been described that employ artificial constructs to promote such complexes and assess the consequences. For example, a protein interaction scheme has been described that examines the effects of a specific phosphorylation event catalyzed by a protein kinase via the provision of an artificial protein binding interface between a modified version of the kinase and a single substrate. We have generalized this type of approach to form the basis for a genetic selection to identify proteins that exert an activity when recruited to a target protein. The assay uses the leucine zipper domains from the mammalian transcription factors Fos and Jun to force the interaction of two proteins. With a target protein fused to the Jun zipper and a library of open reading frames fused to the Fos zipper, we demonstrate this approach in yeast with both a selection to identify membrane-associated proteins and a selection to identify candidate components of the filamentous growth MAP kinase pathway.

Selection of polarized growth sites in yeast

The budding yeast Saccharomyces cerevisiae responds to intracellular and extracellular cues to direct cell growth. Genetic analysis has revealed many components that participate in this process and has provided insight into the mechanisms by which these proteins function. Several of these components, such as the septins, pheromone receptors and GTPase proteins, have homologues in multicellular eukaryotes, suggesting that many aspects of polarized cell growth may be conserved throughout evolution. This review discusses our current understanding of the molecular mechanisms of growth-site selection during the different stages of the yeast life cycle.

Interactions among Rax1p, Rax2p, Bud8p, and Bud9p in marking cortical sites for bipolar bud-site selection in yeast

In the budding yeast Saccharomyces cerevisiae, selection of the bud site determines the axis of polarized cell growth and eventual oriented cell division. Bud sites are selected in specific patterns depending on cell type. These patterns appear to depend on distinct types of marker proteins in the cell cortex; in particular, the bipolar budding of diploid cells depends on persistent landmarks at the birth-scar-distal and -proximal poles that involve the proteins Bud8p and Bud9p, respectively. Rax1p and Rax2p also appear to function specifically in bipolar budding, and we report here a further characterization of these proteins and of their interactions with Bud8p and Bud9p. Rax1p and Rax2p both appear to be integral membrane proteins. Although commonly used programs predict different topologies for Rax2p, glycosylation studies indicate that it has a type I orientation, with its long N-terminal domain in the extracytoplasmic space. Analysis of rax1 and rax2 mutant budding patterns indicates that both proteins are involved in selecting bud sites at both the distal and proximal poles of daughter cells as well as near previously used division sites on mother cells. Consistent with this, GFP-tagged Rax1p and Rax2p were both observed at the distal pole as well as at the division site on both mother and daughter cells; localization to the division sites was persistent through multiple cell cycles. Localization of Rax1p and Rax2p was interdependent, and biochemical studies showed that these proteins could be copurified from yeast. Bud8p and Bud9p could also be copurified with Rax1p, and localization studies provided further evidence of interactions. Localization of Rax1p and Rax2p to the bud tip and distal pole depended on Bud8p, and normal localization of Bud8p was partially dependent on Rax1p and Rax2p. Although localization of Rax1p and Rax2p to the division site did not appear to depend on Bud9p, normal localization of Bud9p appeared largely or entirely dependent on Rax1p and Rax2p. Taken together, the results indicate that Rax1p and Rax2p interact closely with each other and with Bud8p and Bud9p in the establishment and/or maintenance of the cortical landmarks for bipolar budding.

The Ras/protein kinase A pathway acts in parallel with the Mob2/Cbk1 pathway to effect cell cycle progression and proper bud site selection

In Saccharomyces cerevisiae, Ras proteins connect nutrient availability to cell growth through regulation of protein kinase A (PKA) activity. Ras proteins also have PKA-independent functions in mitosis and actin repolarization. We have found that mutations in MOB2 or CBK1 confer a slow-growth phenotype in a ras2Delta background. The slow-growth phenotype of mob2Delta ras2Delta cells results from a G1 delay that is accompanied by an increase in size, suggesting a G1/S role for Ras not previously described. In addition, mob2Delta strains have imprecise bud site selection, a defect exacerbated by deletion of RAS2. Mob2 and Cbk1 act to properly localize Ace2, a transcription factor that directs daughter cell-specific transcription of several genes. The growth and budding phenotypes of the double-deletion strains are Ace2 independent but are suppressed by overexpression of the PKA catalytic subunit, Tpk1. From these observations, we conclude that the PKA pathway and Mob2/Cbk1 act in parallel to determine bud site selection and promote cell cycle progression.

The yeast dynamin-related GTPase Vps1p functions in the organization of the actin cytoskeleton via interaction with Sla1p

Recent studies have suggested that the function of the large GTPase dynamin in endocytosis in mammalian cells may comprise a modulation of actin cytoskeleton. The role of dynamin in actin cytoskeleton organization in the yeast Saccharomyces cerevisiae has remained undefined. In this report, we found that one of the yeast dynamin-related proteins, Vps1p, is required for normal actin cytoskeleton organization. At both permissive and non-permissive temperatures, the vps1 mutants exhibited various degrees of phenotypes commonly associated with actin cytoskeleton defects: depolarized and aggregated actin structures, hypersensitivity to the actin cytoskeleton toxin latrunculin-A, randomized bud site selection and chitin deposition, and impaired efficiency in the internalization of membrane receptors. Over-expression of the GTPase mutants of vps1 also led to actin abnormalities. Consistent with these actin-related defects, Vps1p was found to interact physically, and partially co-localize, with the actin-regulatory protein Sla1p. The normal cellular localization of Sla1p required Vps1p and could be altered by over-expression of a region of Vps1p that was involved in the interaction with Sla1p. The same region also promoted mis-sorting of the vacuolar protein carboxypeptidase Y upon over-expression. These findings suggest that the functions of the dynamin-related protein Vps1p in actin cytoskeleton dynamics and vacuolar protein sorting are probably related to each other.

Specific residues of the GDP/GTP exchange factor Bud5p are involved in establishment of the cell type-specific budding pattern in yeast

Cells of the budding yeast undergo oriented cell division by choosing a specific site for growth depending on their cell type. Haploid a and alpha cells bud in an axial pattern whereas diploid a/alpha cells bud in a bipolar pattern. The Ras-like GTPase Rsr1p/Bud1p, its GDP-GTP exchange factor Bud5p, and its GTPase-activating protein Bud2p are essential for selecting the proper site for polarized growth in all cell types. Here we showed that specific residues at the N terminus and the C terminus of Bud5p were important for bipolar budding, while some residues were involved in both axial and bipolar budding. These bipolar-specific mutations of BUD5 disrupted proper localization of Bud5p in diploid a/alpha cells without affecting Bud5p localization in haploid alpha cells. In contrast, Bud5p expressed in the bud5 mutants defective in both budding patterns failed to localize in all cell types. Thus, these results identify specific residues of Bud5p that are likely to be involved in direct interaction with spatial landmarks, which recruit Bud5p to the proper bud site. Finally, we found a new start codon of BUD5, which extends the open reading frame to 210 bp upstream of the previously estimated start site, thus encoding a polypeptide of 608 amino acid residues. Bud5p with these additional N-terminal residues interacted with Bud8p, a potential bipolar landmark, suggesting that the N-terminal region is necessary for recognition of the spatial cues.

Rax1, a protein required for the establishment of the bipolar budding pattern in yeast

In Saccharomyces cerevisiae, cell type determines two distinct spatial budding patterns. Haploid cells exhibit an axial pattern, whereas diploid cells exhibit a bipolar pattern. Axl1, a member of the insulin-degrading enzyme (IDE) family, is the key morphological determinant for the haploid axial pattern. Here we identified a novel gene, RAX1, specifically required for the bipolar budding pattern. Loss of RAX1 alters the bipolar pattern of axl1 haploids resulting in reversion to the axial pattern, and also alters the bipolar patterns of bud3 and bud4 haploids. However, bud10 rax1 haploids exhibit a random budding pattern, suggesting Bud10 acts as the key proximal landmark in axial budding. Rax1 is required for the localization of Bud8, the distal bipolar budding landmark. Interestingly, Rax1 contains a C-terminal domain possessing some similarity to insulin-related peptides. Our results suggest that Rax1 is necessary for the establishment of the bipolar budding landmark.

Polarized distribution of intracellular components by class V myosins in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae has three classes of myosins corresponding to three actin structures: class I myosin for endocytic actin structure, actin patches; class II myosin for contraction of the actomyosin contractile ring around the bud neck; and class V myosin for transport along a cable-like actin structure (actin cables), extending toward the growing cortex. Myo2p and Myo4p constitute respective class V myosins as the heavy chain and, like class V myosins in other organisms, function as actin-based motors for polarized distribution of organelles and intracellular molecules. Proper distribution of organelles is essential for autonomously replicating organelles that cannot be reproduced de novo, and is also quite important for other organelles to ensure their efficient segregation and proper positioning, even though they can be newly synthesized, such as those derived from endoplasmic reticulum. In the budding yeast, microtubule-based motors play limited roles in the distribution. Instead, the actin-based motor myosins, especially Myo2p, play a major role. Studies on Myo2p have revealed a wide variety of Myo2p cargo and Myo2p-interacting proteins and have established that Myo2p interacts with cargo and transfers it along actin cables. Moreover, recent findings suggest that Myo2p has another way to distribute cargo in that Myo2p conveys the attaching cargo along the actin track. Thus, the myosin have "dual paths" for distribution of a cargo. This dual path mechanism is proposed in the last section of this review.

Chitin scar breaks in aged Saccharomyces cerevisiae

Ageing in budding yeast is not determined by chronological lifespan, but by the number of times an individual cell is capable of dividing, termed its replicative capacity. As cells age they are subject to characteristic cell surface changes. Saccharomyces cerevisiae reproduces asexually by budding and as a consequence of this process both mother and daughter cell retain chitinous scar tissue at the point of cytokinesis. Daughter cells exhibit a frail structure known as the birth scar, while mother cells display a more persistent bud scar. The number of bud scars present on the cell surface is directly related to the number of times a cell has divided and thus constitutes a biomarker for replicative cell age. It has been proposed that the birth scar may be subject to stretching caused by expansion of the daughter cell; however, no previous analysis of the effect of cell age on birth or bud scar size has been reported. This paper provides evidence that scar tissue expands with the cell during growth. It is postulated that symmetrically arranged breaks in the bud scar allow these rigid chitinous structures to expand without compromising cellular integrity.

Septation and cytokinesis in fungi

Cytokinesis is the ultimate step of a cell cycle resulting in the generation of two progeny. Failure of correct cell division may be lethal for both, mother and daughter cells, and thus such a process must be tightly regulated with other events of the cell cycle. Differing solutions to the same problem have been developed in bacteria and plants while cytokinesis in animal and fungal cells is highly similar and requires a contractile ring containing actomyosin. Cytokinesis in fungi can be viewed as a three-stage process: (i) selection of a division site, (ii) orderly assembly of protein complexes, and finally (iii) dynamic events that lead to a constriction of the contractile ring and septum construction. Elaborate mechanisms known as the Mitotic Exit Network (MEN) and the Septation Initiation Network (SIN) have evolved to link these events, particularly the final steps of cytokinesis, with nuclear division. The purpose of this review was to discuss the latest developments in the fungal field and to describe the central known players required for key steps on the road to cell division. Differences in the cytokinesis of yeast-like fungi that result in complete cell separation in contrast to septation which leads to the compartmentalization of fungal hyphae are highlighted.

Interaction between a Ras and a Rho GTPase couples selection of a growth site to the development of cell polarity in yeast

Polarized cell growth requires the coupling of a defined spatial site on the cell cortex to the apparatus that directs the establishment of cell polarity. In the budding yeast Saccharomyces cerevisiae, the Ras-family GTPase Rsr1p/Bud1p and its regulators select the proper site for bud emergence on the cell cortex. The Rho-family GTPase Cdc42p and its associated proteins then establish an axis of polarized growth by triggering an asymmetric organization of the actin cytoskeleton and secretory apparatus at the selected bud site. We explored whether a direct linkage exists between the Rsr1p/Bud1p and Cdc42p GTPases. Here we show specific genetic interactions between RSR1/BUD1 and particular cdc42 mutants defective in polarity establishment. We also show that Cdc42p coimmunoprecipitated with Rsr1p/Bud1p from yeast extracts. In vitro studies indicated a direct interaction between Rsr1p/Bud1p and Cdc42p, which was enhanced by Cdc24p, a guanine nucleotide exchange factor for Cdc42p. Our findings suggest that Cdc42p interacts directly with Rsr1p/Bud1p in vivo, providing a novel mechanism by which direct contact between a Ras-family GTPase and a Rho-family GTPase links the selection of a growth site to polarity establishment.

RAM: a conserved signaling network that regulates Ace2p transcriptional activity and polarized morphogenesis

In Saccharomyces cerevisiae, polarized morphogenesis is critical for bud site selection, bud development, and cell separation. The latter is mediated by Ace2p transcription factor, which controls the daughter cell-specific expression of cell separation genes. Recently, a set of proteins that include Cbk1p kinase, its binding partner Mob2p, Tao3p (Pag1p), and Hym1p were shown to regulate both Ace2p activity and cellular morphogenesis. These proteins seem to form a signaling network, which we designate RAM for regulation of Ace2p activity and cellular morphogenesis. To find additional RAM components, we conducted genetic screens for bilateral mating and cell separation mutants and identified alleles of the PAK-related kinase Kic1p in addition to Cbk1p, Mob2p, Tao3p, and Hym1p. Deletion of each RAM gene resulted in a loss of Ace2p function and caused cell polarity defects that were distinct from formin or polarisome mutants. Two-hybrid and coimmunoprecipitation experiments reveal a complex network of interactions among the RAM proteins, including Cbk1p-Cbk1p, Cbk1p-Kic1p, Kic1p-Tao3p, and Kic1p-Hym1p interactions, in addition to the previously documented Cbk1p-Mob2p and Cbk1p-Tao3p interactions. We also identified a novel leucine-rich repeat-containing protein Sog2p that interacts with Hym1p and Kic1p. Cells lacking Sog2p exhibited the characteristic cell separation and cell morphology defects associated with perturbation in RAM signaling. Each RAM protein localized to cortical sites of growth during both budding and mating pheromone response. Hym1p was Kic1p- and Sog2p-dependent and Sog2p and Kic1p were interdependent for localization, indicating a close functional relationship between these proteins. Only Mob2p and Cbk1p were detectable in the daughter cell nucleus at the end of mitosis. The nuclear localization and kinase activity of the Mob2p-Cbk1p complex were dependent on all other RAM proteins, suggesting that Mob2p-Cbk1p functions late in the RAM network. Our data suggest that the functional architecture of RAM signaling is similar to the S. cerevisiae mitotic exit network and Schizosaccharomyces pombe septation initiation network and is likely conserved among eukaryotes.

A cytokinetic function of Drosophila ORC6 protein resides in a domain distinct from its replication activity

Coordination between separate pathways may be facilitated by the requirements for common protein factors, a finding congruent with the link between proteins regulating DNA replication with other important cellular processes. We report that the smallest of Drosophila origin recognition complex subunits, Orc6, was found in embryos and cell culture localized to the cell membrane and cleavage furrow during cell division as well as in the nucleus. A two-hybrid screen revealed that Orc6 interacts with the Drosophila peanut (pnut), a member of the septin family of proteins important for cell division. This interaction, mediated by a distinct C-terminal domain of Orc6, was substantiated in Drosophila cells by coimmunoprecipitation from extracts and cytological methods. Silencing of Orc6 expression with double-stranded RNA resulted in a formation of multinucleated cells and also reduced DNA replication. Deletion of the C-terminal Orc6-peanut interaction domain and subsequent overexpression of the Orc6 mutant protein resulted in the formation of multinucleated cells that had replicated DNA. This mutant protein does not localize to the membrane or cleavage furrows. These results suggest that Orc6 has evolved a domain critical mainly for cytokinesis.

Identification of p21-activated kinase specificity determinants in budding yeast: a single amino acid substitution imparts Ste20 specificity to Cla4

Two closely related p21-activated kinases from Saccharomyces cerevisiae, Ste20 and Cla4, interact with and are regulated by Cdc42, a small Rho-like GTPase. These kinases are argued to perform a common essential function, based on the observation that the single mutants are viable whereas the double mutant is inviable. Despite having a common upstream regulator and at least one common function, these molecules also have many distinct cellular signaling roles. Ste20 signals upstream of several mitogen-activated protein kinase cascades (e.g., pheromone response, filamentous growth, and high osmolarity), and Cla4 signals during budding and cytokinesis. In order to investigate how these kinases are directed to distinct functions, we sought to identify specificity determinants within Ste20 and Cla4. To this end, we constructed both chimeric fusions and point mutants and tested their ability to perform unique and shared cellular roles. Specificity determinants for both kinases were mapped to the C-terminal kinase domains. Remarkably, the substitution of a single amino acid, threonine 818, from Ste20 into an otherwise wild-type Cla4, Cla4D772T, conferred the ability to perform many Ste20-specific functions.

The upstream regulator, Rsr1p, and downstream effectors, Gic1p and Gic2p, of the Cdc42p small GTPase coordinately regulate initiation of budding in Saccharomyces cerevisiae

BACKGROUND

Cdc42p, a Rho family small GTPase, is essential for budding initiation in the yeast Saccharomyces cerevisiae. The homologous proteins Gic1p and Gic2p (Gic1/2p) are effectors of Cdc42p, but their precise functions remain unknown. Rsr1p/Bud1p is a Ras family small GTPase that controls the selection of the budding site. Previous observations suggested that Rsr1p-GTP recruits Cdc24p, a GDP/GTP exchange factor for Cdc42p, at the incipient bud site. However, this model only addresses how Rsr1p determines the budding site, because the rsr1 mutant normally initiates budding.

RESULTS

Here we show that a rsr1 gic1 gic2 mutant fails to initiate budding, resulting in unbudded, large, and multinucleated cells. Expression of a dominant active or dominant negative mutant of RSR1 also inhibited the growth of the gic1 gic2 mutant, suggesting that cycling of Rsr1p between the GTP- and GDP-bound forms is required for budding initiation in the gic1 gic2 mutant. Among the mutations in effectors of CDC42, only the gic1 gic2 mutation demonstrated a synthetic lethal interaction with rsr1. Increased gene dosage of CDC42 suppressed defects in budding initiation of rsr1 gic1 gic2 mutants containing additional mutations in other effectors of CDC42, including BNI1, CLA4 or STE20. The polarized localization of Bni1p-GFP (green fluorescent protein) and Cla4p-GFP was lost after depletion of Gic1p in the rsr1 gic2 mutant.

CONCLUSION

We propose that Gic1/2p may stabilize or maintain a complex consisting of Cdc42p-GTP and its effectors at the budding site, which are assembled by the action of the Rsr1p-Cdc24p system.

Cytokinesis: relative alignment of the cell division apparatus and the mitotic spindle

The cell division apparatus is assembled at different stages of the cell cycle in different eukaryotic organisms. Mechanisms exist in all organisms, however, to ensure that the cell division apparatus and the mitotic spindle are aligned perpendicular to each other. Such an alignment ensures that each daughter cell receives a nucleus and that the cell division apparatus does not cleave and destroy the genetic material. The interaction(s) of astral microtubules with the cell cortex appears to play an important role in establishing perpendicularity between chromosome segregation and cell division machinery.

The Glc7p-interacting protein Bud14p attenuates polarized growth, pheromone response, and filamentous growth in Saccharomyces cerevisiae

A genetic selection in Saccharomyces cerevisiae for mutants that stimulate the mating pathway uncovered a mutant that had a hyperactive pheromone response pathway and also had hyperpolarized growth. Cloning and segregation analysis demonstrated that BUD14 was the affected gene. Disruption of BUD14 in wild-type cells caused mild stimulation of pheromone response pathway reporters, an increase in sensitivity to mating factor, and a hyperelongated shmoo morphology. The bud14 mutant also had hyperfilamentous growth. Consistent with a role in the control of cell polarity, a Bud14p-green fluorescent protein fusion was localized to sites of polarized growth in the cell. Bud14p shared morphogenetic functions with the Ste20p and Bni1p proteins as well as with the type 1 phosphatase Glc7p. The genetic interactions between BUD14 and GLC7 suggested a role for Glc7p in filamentous growth, and Glc7p was found to have a positive function in filamentous growth in yeast.

Iqg1p links spatial and secretion landmarks to polarity and cytokinesis

Cytokinesis requires the polarization of the actin cytoskeleton, the secretion machinery, and the correct positioning of the division axis. Budding yeast cells commit to their cytokinesis plane by choosing a bud site and polarizing their growth. Iqg1p (Cyk1p) was previously implicated in cytokinesis (Epp and Chant, 1997; Lippincott and Li, 1998; Osman and Cerione, 1998), as well as in the establishment of polarity and protein trafficking (Osman and Cerione, 1998). To better understand how Iqg1p influences these processes, we performed a two-hybrid screen and identified the spatial landmark Bud4p as a binding partner. Iqg1p can be coimmunoprecipitated with Bud4p, and Bud4p requires Iqg1p for its proper localization. Iqg1p also appears to specify axial bud-site selection and mediates the proper localization of the septin, Cdc12p, as well as binds and helps localize the secretion landmark, Sec3p. The double mutants iqg1Deltasec3Delta and bud4Deltasec3Delta display defects in polarity, budding pattern and cytokinesis, and electron microscopic studies reveal that these cells have aberrant septal deposition. Taken together, these findings suggest that Iqg1p recruits landmark proteins to form a targeting patch that coordinates axial budding with cytokinesis.

The roles of bud-site-selection proteins during haploid invasive growth in yeast

In haploid strains of Saccharomyces cerevisiae, glucose depletion causes invasive growth, a foraging response that requires a change in budding pattern from axial to unipolar-distal. To begin to address how glucose influences budding pattern in the haploid cell, we examined the roles of bud-site-selection proteins in invasive growth. We found that proteins required for bipolar budding in diploid cells were required for haploid invasive growth. In particular, the Bud8p protein, which marks and directs bud emergence to the distal pole of diploid cells, was localized to the distal pole of haploid cells. In response to glucose limitation, Bud8p was required for the localization of the incipient bud site marker Bud2p to the distal pole. Three of the four known proteins required for axial budding, Bud3p, Bud4p, and Axl2p, were expressed and localized appropriately in glucose-limiting conditions. However, a fourth axial budding determinant, Axl1p, was absent in filamentous cells, and its abundance was controlled by glucose availability and the protein kinase Snf1p. In the bud8 mutant in glucose-limiting conditions, apical growth and bud site selection were uncoupled processes. Finally, we report that diploid cells starved for glucose also initiate the filamentous growth response.

Subcellular localization of Axl1, the cell type-specific regulator of polarity

Bud-site selection in yeast offers an attractive system for studying cell polarity and asymmetric division. Haploids divide in an axial pattern, whereas diploids divide in a bipolar pattern. AXL1 is expressed in haploids but not diploids, and ectopic expression of AXL1 in diploids converts their bipolar budding pattern to an axial pattern. How Axl1 acts as a switch between the bipolar and axial patterns is not understood. Here we report that Axl1 localizes to the mother-bud neck and division site remnants of haploids. Axl1 is absent from diploids. Axl1 colocalizes with Bud3, Bud4, and Bud10, components of the axial landmark structure. This localization suggests that Axl1 couples the axial landmark with downstream polarity establishment factors. Consistent with such a role, Axl1 associated biochemically with Bud4 and Bud5. Genetic evidence suggests that Axl1 works with Bud3 and Bud4 to promote the activity of the Bud10 membrane protein. Given Axl1's suggested role in morphogenesis and cell fusion during mating, we also examined its localization during this process. Axl1 redistributes independently of the axial landmark to a tight cell surface dot at the tip of each mating projection. These dots are rapidly lost as prezygotes form.

Localization of the Rsr1/Bud1 GTPase involved in selection of a proper growth site in yeast

Yeast cells organize their actin cytoskeleton in a highly polarized manner during vegetative growth. The Ras-like GTPase Rsr1/Bud1 and its regulators are required for selection of a specific site for growth. Here we showed that Rsr1/Bud1 was broadly distributed on the plasma membrane and highly concentrated at the incipient bud site and polarized growth sites. We also showed that localization of Cdc24, a guanine nucleotide exchange factor for the Cdc42 GTPase, to the proper bud site was dependent on Rsr1/Bud1. Surprisingly, Rsr1/Bud1 also localized to intracellular membranes. A mutation in the lysine repeat in the hypervariable region of Rsr1/Bud1 specifically abolished its plasma membrane localization, whereas a mutation at the CAAX motif eliminated both plasma membrane and internal membrane association of Rsr1/Bud1. Thus the lysine repeat and the CAAX motif of Rsr1/Bud1 are important for its localization to the plasma membrane and to the polarized growth sites. This localization of Rsr1/Bud1 is essential for its function in proper bud site selection because both mutations resulted in random bud site selection.

Mechanisms for targeting of theSaccharomyces cerevisiaeGPI-anchored cell wall protein Crh2p to polarised growth sites

The cell wall is an essential structure that preserves the osmotic integrity of fungal cells and determines cellular morphology during developmental programs. The high number of different wall components demands a variety of processes to deliver precursors and synthetic proteins to the proper location at the right time for wall development and modification. Here,the specificity of the mechanisms that regulate the temporal and spatial localisation of cell wall proteins to sites of polarised growth in Saccharomyces cerevisiae is investigated. For this purpose, the localisation of Crh2p, a cell wall glycosylphosphatidylinositol (GPI)-anchored mannoprotein that we have recently described as involved in cell wall construction and localised to polarised growth sites, was followed using a Crh2p-GFP fusion protein. Crh2p distribution was studied in several genetic backgrounds affected in different steps of the cell polarity establishment machinery or/and bud morphogenesis. Crh2p is localised at the mother-bud neck in bud1 cells following the random budding pattern characteristic of this mutant. The Crh2p distribution was greatly altered in a cdc42-1mutant, indicating complete dependence on an organised actin cytoskeleton for polarised Crh2p distribution. The usual deposition of Crh2p in a ring at the base of growing buds was lacking in cdc10-11 cells growing under restrictive temperature conditions, whereas Crh2p deposition at the septum region was absent in both cdc10-11 and cdc15-lyt1 cells. These results point to the dependence of Crh2p localisation at the bud-neck on both septins and septum integrity. Furthermore, in the absence of Bni4p, a scaffold protein involved in the targeting of the chitin synthase III complex to the bud neck, Crh2p was not longer found at the neck in large-budded cells undergoing cytokinesis. Finally, Crh2p was not properly localised in cells deleted in CHS5, which encodes a protein involved in the transport of Chs3p, and was completely mislocalised in sbe2/sbe22 mutants,suggesting that the transport systems for Chs3p and Crh2p are to a certain extent coincident. The transport of other GPI-cell wall proteins, such as Cwp1p, however, does not depend on these systems as the localisation of the latter protein was not affected in either of these mutants.

Regulation of G protein-initiated signal transduction in yeast: paradigms and principles

All cells have the capacity to evoke appropriate and measured responses to signal molecules (such as peptide hormones), environmental changes, and other external stimuli. Tremendous progress has been made in identifying the proteins that mediate cellular response to such signals and in elucidating how events at the cell surface are linked to subsequent biochemical changes in the cytoplasm and nucleus. An emerging area of investigation concerns how signaling components are assembled and regulated (both spatially and temporally), so as to control properly the specificity and intensity of a given signaling pathway. A related question under intensive study is how the action of an individual signaling pathway is integrated with (or insulated from) other pathways to constitute larger networks that control overall cell behavior appropriately. This review describes the signal transduction pathway used by budding yeast (Saccharomyces cerevisiae) to respond to its peptide mating pheromones. This pathway is comprised by receptors, a heterotrimeric G protein, and a protein kinase cascade all remarkably similar to counterparts in multicellular organisms. The primary focus of this review, however, is recent advances that have been made, using primarily genetic methods, in identifying molecules responsible for regulation of the action of the components of this signaling pathway. Just as many of the constituent proteins of this pathway and their interrelationships were first identified in yeast, the functions of some of these regulators have clearly been conserved in metazoans, and others will likely serve as additional models for molecules that carry out analogous roles in higher organisms.

Fast 100-nm resolution three-dimensional microscope reveals structural plasticity of mitochondria in live yeast

By introducing beam-scanning multifocal multiphoton 4Pi-confocal microscopy, we have attained fast fluorescence imaging of live cells with axial super resolution. Rapid scanning of up to 64 pairs of interfering high-angle fields and subsequent confocal detection enabled us to perform three to five times finer optical sectioning than confocal microscopy. In conjunction with nonlinear image restoration, we demonstrate, to our knowledge for the first time, three-dimensional imaging of live eukaryotic cells at an equilateral resolution of approximately 100 nm. This imaging mode allowed us to reveal the morphology and size of the green fluorescent protein-labeled mitochondrial compartment of live Saccharomyces cerevisiae (bakers' yeast) growing on different carbon sources. Our studies show that mitochondria of cells grown on medium containing glycerol as the only carbon source, as opposed to glucose-grown cells, exhibit a strongly branched tubular reticulum. We determine the average tubular diameter and find that it increases from 339 +/- 5 nm to 360 +/- 4 nm when changing from glucose to glycerol, that is, from a fermentable to a nonfermentable carbon source. Moreover, this change is associated with a 2.8-fold increase of the surface of the reticulum, resulting in an average increase in volume of the mitochondrial compartment by a factor of 3.0 +/- 0.2.