Autoradiographic study of mannan incorporation into the growing cell walls of Saccharomyces cerevisiae

Study of impacts of two types of cellular aging on the yeast bud morphogenesis

Understanding the mechanisms of the cellular aging processes is crucial for attempting to extend organismal lifespan and for studying age-related degenerative diseases. Yeast cells divide through budding, providing a classical biological model for studying cellular aging. With their powerful genetics, relatively short cell cycle, and well-established signaling pathways also found in animals, yeast cells offer valuable insights into the aging process. Recent experiments suggested the existence of two aging modes in yeast characterized by nucleolar and mitochondrial declines, respectively. By analyzing experimental data, this study shows that cells evolving into those two aging modes behave differently when they are young. While buds grow linearly in both modes, cells that consistently generate spherical buds throughout their lifespan demonstrate greater efficacy in controlling bud size and growth rate at young ages. A three-dimensional multiscale chemical-mechanical model was developed and used to suggest and test hypothesized impacts of aging on bud morphogenesis. Experimentally calibrated model simulations showed that during the early stage of budding, tubular bud shape in one aging mode could be generated by locally inserting new materials at the bud tip, a process guided by the polarized Cdc42 signal. Furthermore, the aspect ratio of the tubular bud could be stabilized during the late stage as observed in experiments in this work. The model simulation results suggest that the localization of new cell surface material insertion, regulated by chemical signal polarization, could be weakened due to cellular aging in yeast and other cell types, leading to the change and stabilization of the bud aspect ratio.

Study of Impacts of Two Types of Cellular Aging on the Yeast Bud Morphogenesis

Understanding the mechanisms of cellular aging processes is crucial for attempting to extend organismal lifespan and for studying age-related degenerative diseases. Yeast cells divide through budding, providing a classical biological model for studying cellular aging. With their powerful genetics, relatively short lifespan and well-established signaling pathways also found in animals, yeast cells offer valuable insights into the aging process. Recent experiments suggested the existence of two aging modes in yeast characterized by nucleolar and mitochondrial declines, respectively. In this study, by analyzing experimental data it was shown that cells evolving into those two aging modes behave differently when they are young. While buds grow linearly in both modes, cells that consistently generate spherical buds throughout their lifespan demonstrate greater efficacy in controlling bud size and growth rate at young ages. A three-dimensional chemical-mechanical model was developed and used to suggest and test hypothesized mechanisms of bud morphogenesis during aging. Experimentally calibrated simulations showed that tubular bud shape in one aging mode could be generated by locally inserting new materials at the bud tip guided by the polarized Cdc42 signal during the early stage of budding. Furthermore, the aspect ratio of the tubular bud could be stabilized during the late stage, as observed in experiments, through a reduction on the new cell surface material insertion or an expansion of the polarization site. Thus model simulations suggest the maintenance of new cell surface material insertion or chemical signal polarization could be weakened due to cellular aging in yeast and other cell types.

The mitotic exit network regulates the spatiotemporal activity of Cdc42 to maintain cell size

During G1 in budding yeast, the Cdc42 GTPase establishes a polar front, along which actin is recruited to direct secretion for bud formation. Cdc42 localizes at the bud cortex and then redistributes between mother and daughter in anaphase. The molecular mechanisms that terminate Cdc42 bud-localized activity during mitosis are poorly understood. We demonstrate that the activity of the Cdc14 phosphatase, released through the mitotic exit network, is required for Cdc42 redistribution between mother and bud. Induced Cdc14 nucleolar release results in premature Cdc42 redistribution between mother and bud. Inhibition of Cdc14 causes persistence of Cdc42 bud localization, which perturbs normal cell size and spindle positioning. Bem3, a Cdc42 GAP, binds Cdc14 and is dephosphorylated at late anaphase in a Cdc14-dependent manner. We propose that Cdc14 dephosphorylates and activates Bem3 to allow Cdc42 inactivation and redistribution. Our results uncover a mechanism through which Cdc14 regulates the spatiotemporal activity of Cdc42 to maintain normal cell size at cytokinesis.

Distinct segregation patterns of yeast cell-peripheral proteins uncovered by a method for protein segregatome analysis

Protein segregation contributes to various cellular processes such as polarization, differentiation, and aging. However, the difficulty in global determination of protein segregation hampers our understanding of its mechanisms and physiological roles. Here, by developing a quantitative proteomics technique, we globally monitored segregation of preexisting and newly synthesized proteins during cell division of budding yeast, and identified crucial domains that determine the segregation of cell-peripheral proteins. Remarkably, the proteomic and subsequent microscopic analyses demonstrated that the flow through the bud neck of the proteins that harbor both endoplasmic reticulum (ER) membrane-spanning and plasma membrane (PM)-binding domains is not restricted by the previously suggested ER membrane or PM diffusion barriers but by septin-mediated partitioning of the PM-associated ER (pmaER). Furthermore, the proteomic analysis revealed that although the PM-spanning t-SNARE Sso2 was retained in mother cells, its paralog Sso1 unexpectedly showed symmetric localization. We found that the transport of Sso1 to buds was required for enhancement of polarized cell growth and resistance to cell-wall stress. Taken together, these data resolve long-standing questions about septin-mediated compartmentalization of the cell periphery, and provide new mechanistic insights into the segregation of cell-periphery proteins and their cellular functions.

Prolonged cyclin-dependent kinase inhibition results in septin perturbations during return to growth and mitosis

By investigating how yeast cells coordinate polarity and division in a special type of cell division called return to growth, Gihana et al. discover that although checkpoints are normally beneficial, prolonged activation of the morphogenesis checkpoint is instead detrimental to the cell.

We investigated how Saccharomyces cerevisiae coordinate polarization, budding, and anaphase during a unique developmental program called return to growth (RTG) in which cells in meiosis return to mitosis upon nutrient shift. Cells reentering mitosis from prophase I deviate from the normal cell cycle by budding in G2 instead of G1. We found that cells do not maintain the bipolar budding pattern, a characteristic of diploid cells. Furthermore, strict temporal regulation of M-phase cyclin-dependent kinase (CDK; M-CDK) is important for polarity establishment and morphogenesis. Cells with premature M-CDK activity caused by loss of checkpoint kinase Swe1 failed to polarize and underwent anaphase without budding. Mutants with increased Swe1-dependent M-CDK inhibition showed additional or more penetrant phenotypes in RTG than mitosis, including elongated buds, multiple buds, spindle mispositioning, and septin perturbation. Surprisingly, the enhanced and additional phenotypes were not exclusive to RTG but also occurred with prolonged Swe1-dependent CDK inhibition in mitosis. Our analysis reveals that prolonged activation of the Swe1-dependent checkpoint can be detrimental instead of beneficial.

The bud tip is the cellular hot spot of protein secretion in yeasts

Yeasts are valuable hosts for recombinant protein production. Among them, Pichia pastoris is frequently used for production of secreted proteins, and much effort was made to improve the secretion efficiency of this expression platform. However, the knowledge on the secretion machinery is mainly based on studies in Saccharomyces cerevisiae. Therefore, it is of great interest for targeted improvement of the system to learn more about the secretion process in P. pastoris. Using human serum albumin, a protein which is produced in high quantities in P. pastoris, we show here the secretion pathway of this protein. During passage of the secretory route, the recombinant protein is mainly localized in the endoplasmic reticulum (ER) and in COPII vesicles, and is inherited to the daughter cell via the perinuclear ER. The final release to the cell exterior occurs at the bud, initiating at the bud tip and later spreading over the entire bud surface. The same polarized secretion pattern was observed for a recombinant antibody light chain and the native secretory protein Epx1 of P. pastoris. Clarifying the point of release of secretory proteins will have major impact on engineering the secretory pathway of P. pastoris and other budding yeasts.

Cell polarization and cytokinesis in budding yeast

Asymmetric cell division, which includes cell polarization and cytokinesis, is essential for generating cell diversity during development. The budding yeast Saccharomyces cerevisiae reproduces by asymmetric cell division, and has thus served as an attractive model for unraveling the general principles of eukaryotic cell polarization and cytokinesis. Polarity development requires G-protein signaling, cytoskeletal polarization, and exocytosis, whereas cytokinesis requires concerted actions of a contractile actomyosin ring and targeted membrane deposition. In this chapter, we discuss the mechanics and spatial control of polarity development and cytokinesis, emphasizing the key concepts, mechanisms, and emerging questions in the field.

Morphogenesis and the cell cycle

Studies of the processes leading to the construction of a bud and its separation from the mother cell in Saccharomyces cerevisiae have provided foundational paradigms for the mechanisms of polarity establishment, cytoskeletal organization, and cytokinesis. Here we review our current understanding of how these morphogenetic events occur and how they are controlled by the cell-cycle-regulatory cyclin-CDK system. In addition, defects in morphogenesis provide signals that feed back on the cyclin-CDK system, and we review what is known regarding regulation of cell-cycle progression in response to such defects, primarily acting through the kinase Swe1p. The bidirectional communication between morphogenesis and the cell cycle is crucial for successful proliferation, and its study has illuminated many elegant and often unexpected regulatory mechanisms. Despite considerable progress, however, many of the most puzzling mysteries in this field remain to be resolved.

A microfluidic device to acquire high-magnification microphotographs of yeast cells

BACKGROUND

Yeast cell morphology was investigated to reveal the molecular mechanisms of cell morphogenesis and to identify key factors of other processes such as cell cycle progression. We recently developed a semi-automatic image processing program called CalMorph, which allows us to quantitatively analyze yeast cell morphology with the 501 parameters as biological traits and uncover statistical relationships between cell morphological phenotypes and genotypes. However, the current semi-automatic method is not suitable for morphological analysis of large-scale yeast mutants for the reliable prediction of gene functions because of its low-throughput especially at the manual image-acquiring process.

RESULTS

In this study, we developed a microfluidic chip designed to acquire successive microscopic images of yeast cells suitable for CalMorph image analysis. With the microfluidic chip, the morphology of living cells and morphological changes that occur during the cell cycle were successfully characterized.

CONCLUSION

The microfluidic chip enabled us to acquire the images faster than the conventional method. We speculate that the use of microfluidic chip is effective in acquiring images of large-scale for automated analysis of yeast strains.

A temperature-sensitive dcw1 mutant of Saccharomyces cerevisiae is cell cycle arrested with small buds which have aberrant cell walls

Dcw1p and Dfg5p in Saccharomyces cerevisiae are homologous proteins that were previously shown to be involved in cell wall biogenesis and to be essential for growth. Dcw1p was found to be a glycosylphosphatidylinositol-anchored membrane protein. To investigate the roles of these proteins in cell wall biogenesis and cell growth, we constructed mutant alleles of DCW1 by random mutagenesis, introduced them into a Deltadcw1 Deltadfg5 background, and isolated a temperature-sensitive mutant, DC61 (dcw1-3 Deltadfg5). When DC61 cells were incubated at 37 degrees C, most cells had small buds, with areas less than 20% of those of the mother cells. This result indicates that DC61 cells arrest growth with small buds at 37 degrees C. At 37 degrees C, fewer DC61 cells had 1N DNA content and most of them still had a single nucleus located apart from the bud neck. In addition, in DC61 cells incubated at 37 degrees C, bipolar spindles were not formed. These results indicate that DC61 cells, when incubated at 37 degrees C, are cell cycle arrested after DNA replication and prior to the separation of spindle pole bodies. The small buds of DC61 accumulated chitin in the bud cortex, and some of them were lysed, which indicates that they had aberrant cell walls. A temperature-sensitive dfg5 mutant, DF66 (Deltadcw1 dfg5-29), showed similar phenotypes. DCW1 and DFG5 mRNA levels peaked in the G1 and S phases, respectively. These results indicate that Dcw1p and Dfg5p are involved in bud formation through their involvement in biogenesis of the bud cell wall.

Saccharomyces cerevisiae Ats1p interacts with Nap1p, a cytoplasmic protein that controls bud morphogenesis

Saccharomyces cerevisiae ATS1 (alpha-tubulin suppressor 1) was originally identified as a high-copy suppressor of class two alpha-tubulin mutations and was proposed to have a regulatory role in coordinating the microtubule state with the cell cycle. Here, we show that Ats1p interacts with Nap1p, a cytoplasmic protein that regulates the activity of the Cdc28p/Clb2p complex. Loss of Nap1p results in a delayed switch from polar to isotropic bud growth. The delayed switch results in elongated buds. Nap1p and Ats1p interact in two-hybrid and co-immunoprecipitation assays. Both nap1Delta and ats1Delta cells have a Clb2p-dependent elongated bud morphology. Deletion of ATS1 partially suppresses the elongated bud morphology and benomyl resistance of nap1Delta mutants. Our results suggest Ats1p might regulate coordination of the microtubule state with the cell cycle through an interaction with Nap1p.

Structure and biosynthesis of fungal cell walls: Methodological approaches

Fungal cell walls possess a characteristic chemical composition differentiating fungal cells from other cell types. For this reason, the mechanisms involved in cell-wall formation represent a potential target for selective antifungal drugs. Understanding the structure and biosynthesis of fungal cell walls opens the ways for design of effective drugs for treating fungal diseases. This article reviews the history methods employed in chemical and structural analysis of fungal cell walls and in studies concerning their formation.

Membrane trafficking in cytokinesis

Until recently, two distinct types of cytokinesis were thought to be responsible for the division of plant and animal cells. Plant cells divide through the formation of a membrane plate between the daughter cells, while animal cells divide by the constriction of a cortical actin-based ring around the cell. However, accumulating evidence suggests that the two mechanisms may have more in common than previously thought. In this review we will focus on recent developments that raise the possibility of unexpected similarities between the final steps in cytokinesis in animal and plant cells.

Spatial and temporal sequence of capsule construction in Cryptococcus neoformans

The pathogenic yeast Cryptococcus neoformans is distinguished by an extensive polysaccharide capsule, which impedes host defences and is absolutely required for fungal virulence. Despite the biological importance of the capsule, nothing is known about how it is assembled. Substantial capsule growth occurs in two distinct situations relevant to cryptococcal pathogenesis: formation of new buds and induction of capsule on mature cells. We developed pulse-chase protocols to examine these events in a dynamic way using a variety of microscopy techniques. We show that the capsule overlying buds is newly synthesized and differs physically from the corresponding parental material. New capsule formed by mature cells upon induction of synthesis is added at the inner aspect of the existing structure, displacing pre-existing material outwards. Surprisingly, new polysaccharide material is also deposited throughout the capsule, yielding a progressively denser structure. These results yield the first model of capsule synthesis and open new lines of investigation into the underlying mechanisms.

Susceptibility of individual cells of Saccharomyces cerevisiae to the killer toxin K1

The susceptibility of sensitive yeast to killer toxins is known to depend on various factors, such as the selected killer toxin, the exposed yeast strain, its growth phase and the state of culture under given experimental conditions. The aim of this paper was to find whether individual cells from one culture are equally susceptible to the impact of the killer toxin. For this purpose the rhodamine B assay in a modified form was used. In order to observe the fate of individual cell the method of fluorescence video microscopy with a digital picture analysis was applied. Four selected groups of specific cells (with no, small, medium, and large bud, respectively) were investigated. Different sensitivity of Saccharomyces cerevisiae cells to the killer toxin K1 was observed in these cell groups. The most susceptible appeared to be the cells which were in S-phase (cells with the small buds); the least susceptible were the M-phase cells with large buds. The enhanced susceptibility in S-phase results probably from coincidence in higher porosity of the cell wall, accumulation of surface receptors, and enlarged growth activity at the surface cell structures.

Dynamic localization and function of Bni1p at the sites of directed growth in Saccharomyces cerevisiae

Formin homology (FH) proteins are implicated in cell polarization and cytokinesis through actin organization. There are two FH proteins in the yeast Saccharomyces cerevisiae, Bni1p and Bnr1p. Bni1p physically interacts with Rho family small G proteins (Rho1p and Cdc42p), actin, two actin-binding proteins (profilin and Bud6p), and a polarity protein (Spa2p). Here we analyzed the in vivo localization of Bni1p by using a time-lapse imaging system and investigated the regulatory mechanisms of Bni1p localization and function in relation to these interacting proteins. Bni1p fused with green fluorescent protein localized to the sites of cell growth throughout the cell cycle. In a small-budded cell, Bni1p moved along the bud cortex. This dynamic localization of Bni1p coincided with the apparent site of bud growth. A bni1-disrupted cell showed a defect in directed growth to the pre-bud site and to the bud tip (apical growth), causing its abnormally spherical cell shape and thick bud neck. Bni1p localization at the bud tips was absolutely dependent on Cdc42p, largely dependent on Spa2p and actin filaments, and partly dependent on Bud6p, but scarcely dependent on polarized cortical actin patches or Rho1p. These results indicate that Bni1p regulates polarized growth within the bud through its unique and dynamic pattern of localization, dependent on multiple factors, including Cdc42p, Spa2p, Bud6p, and the actin cytoskeleton.

Exocyst is involved in cystogenesis and tubulogenesis and acts by modulating synthesis and delivery of basolateral plasma membrane and secretory proteins

Epithelial cyst and tubule formation are critical processes that involve transient, highly choreographed changes in cell polarity. Factors controlling these changes in polarity are largely unknown. One candidate factor is the highly conserved eight-member protein complex called the exocyst. We show that during tubulogenesis in an in vitro model system the exocyst relocalized along growing tubules consistent with changes in cell polarity. In yeast, the exocyst subunit Sec10p is a crucial component linking polarized exocytic vesicles with the rest of the exocyst complex and, ultimately, the plasma membrane. When the exocyst subunit human Sec10 was exogenously expressed in epithelial Madin-Darby canine kidney cells, there was a selective increase in the synthesis and delivery of apical and basolateral secretory proteins and a basolateral plasma membrane protein, but not an apical plasma membrane protein. Overexpression of human Sec10 resulted in more efficient and rapid cyst formation and increased tubule formation upon stimulation with hepatocyte growth factor. We conclude that the exocyst plays a central role in the development of epithelial cysts and tubules.

Septin-dependent assembly of a cell cycle-regulatory module in Saccharomyces cerevisiae

Saccharomyces cerevisiae septin mutants have pleiotropic defects, which include the formation of abnormally elongated buds. This bud morphology results at least in part from a cell cycle delay imposed by the Cdc28p-inhibitory kinase Swe1p. Mutations in three other genes (GIN4, encoding a kinase related to the Schizosaccharomyces pombe mitotic inducer Nim1p; CLA4, encoding a p21-activated kinase; and NAP1, encoding a Clb2p-interacting protein) also produce perturbations of septin organization associated with an Swe1p-dependent cell cycle delay. The effects of gin4, cla4, and nap1 mutations are additive, indicating that these proteins promote normal septin organization through pathways that are at least partially independent. In contrast, mutations affecting the other two Nim1p-related kinases in S. cerevisiae, Hsl1p and Kcc4p, produce no detectable effect on septin organization. However, deletion of HSL1, but not of KCC4, did produce a cell cycle delay under some conditions; this delay appears to reflect a direct role of Hsl1p in the regulation of Swe1p. As shown previously, Swe1p plays a central role in the morphogenesis checkpoint that delays the cell cycle in response to defects in bud formation. Swe1p is localized to the nucleus and to the daughter side of the mother bud neck prior to its degradation in G(2)/M phase. Both the neck localization of Swe1p and its degradation require Hsl1p and its binding partner Hsl7p, both of which colocalize with Swe1p at the daughter side of the neck. This localization is lost in mutants with perturbed septin organization, suggesting that the release of Hsl1p and Hsl7p from the neck may reduce their ability to inactivate Swe1p and thus contribute to the G(2) delay observed in such mutants. In contrast, treatments that perturb actin organization have little effect on Hsl1p and Hsl7p localization, suggesting that such treatments must stabilize Swe1p by another mechanism. The apparent dependence of Swe1p degradation on localization of the Hsl1p-Hsl7p-Swe1p module to a site that exists only in budded cells may constitute a mechanism for deactivating the morphogenesis checkpoint when it is no longer needed (i.e., after a bud has formed).

Role of actin and Myo2p in polarized secretion and growth of Saccharomyces cerevisiae

We examined the role of the actin cytoskeleton in secretion in Saccharomyces cerevisiae with the use of several quantitative assays, including time-lapse video microscopy of cell surface growth in individual living cells. In latrunculin, which depolymerizes filamentous actin, cell surface growth was completely depolarized but still occurred, albeit at a reduced level. Thus, filamentous actin is necessary for polarized secretion but not for secretion per se. Consistent with this conclusion, latrunculin caused vesicles to accumulate at random positions throughout the cell. Cortical actin patches cluster at locations that correlate with sites of polarized secretion. However, we found that actin patch polarization is not necessary for polarized secretion because a mutant, bee1Delta(las17Delta), which completely lacks actin patch polarization, displayed polarized growth. In contrast, a mutant lacking actin cables, tpm1-2 tpm2Delta, had a severe defect in polarized growth. The yeast class V myosin Myo2p is hypothesized to mediate polarized secretion. A mutation in the motor domain of Myo2p, myo2-66, caused growth to be depolarized but with only a partial decrease in the level of overall growth. This effect is similar to that of latrunculin, suggesting that Myo2p interacts with filamentous actin. However, inhibition of Myo2p function by expression of its tail domain completely abolished growth.

Signaling to the actin cytoskeleton

The actin cytoskeleton is a highly dynamic network composed of actin polymers and a large variety of associated proteins. The main functions of the actin cytoskeleton are to mediate cell motility and cell shape changes during the cell cycle and in response to extracellular stimuli, to organize the cytoplasm, and to generate mechanical forces within the cell. The reshaping and functions of the actin cytoskeleton are regulated by signaling pathways. Here we broadly review the actin cytoskeleton and the signaling pathways that regulate it. We place heavy emphasis on the yeast actin cytoskeleton.

Sec3p is a spatial landmark for polarized secretion in budding yeast

Exocytosis in yeast occurs at plasma membrane subdomains whose locations vary with the cell cycle, but the primary protein determinants of these sites are unknown. A functional fusion of Sec3 protein with green fluorescent protein (Sec3-GFP) localizes to the site of polarized exocytosis for each cell-cycle stage, where it colocalizes with Sec4p and Sec8p. Sec3-GFP localization is independent of secretory pathway function, of the actin and septin cytoskeletons, and of the polarity establishment proteins. We propose that Sec3p is a spatial landmark defining sites of exocytosis. Polarized secretion would result from the coupling of actin-dependent vesicle targeting with Sec3p-dependent establishment of the vesicle fusion site.

A small conserved domain in the yeast Spa2p is necessary and sufficient for its polarized localization

SPA2 encodes a yeast protein that is one of the first proteins to localize to sites of polarized growth, such as the shmoo tip and the incipient bud. The dynamics and requirements for Spa2p localization in living cells are examined using Spa2p green fluorescent protein fusions. Spa2p localizes to one edge of unbudded cells and subsequently is observable in the bud tip. Finally, during cytokinesis Spa2p is present as a ring at the mother-daughter bud neck. The bud emergence mutants bem1 and bem2 and mutants defective in the septins do not affect Spa2p localization to the bud tip. Strikingly, a small domain of Spa2p comprised of 150 amino acids is necessary and sufficient for localization to sites of polarized growth. This localization domain and the amino terminus of Spa2p are essential for its function in mating. Searching the yeast genome database revealed a previously uncharacterized protein which we name, Sph1p (a2p omolog), with significant homology to the localization domain and amino terminus of Spa2p. This protein also localizes to sites of polarized growth in budding and mating cells. SPH1, which is similar to SPA2, is required for bipolar budding and plays a role in shmoo formation. Overexpression of either Spa2p or Sph1p can block the localization of either protein fused to green fluorescent protein, suggesting that both Spa2p and Sph1p bind to and are localized by the same component. The identification of a 150-amino acid domain necessary and sufficient for localization of Spa2p to sites of polarized growth and the existence of this domain in another yeast protein Sph1p suggest that the early localization of these proteins may be mediated by a receptor that recognizes this small domain.

Sec3p is involved in secretion and morphogenesis in Saccharomyces cerevisiae

Two new temperature-sensitive alleles of SEC3, 1 of 10 late-acting SEC genes required for targeting or fusion of post-Golgi secretory vesicles to the plasma membrane in Saccharomyces cerevisiae, were isolated in a screen for temperature-sensitive secretory mutants that are synthetically lethal with sec4-8. The new sec3 alleles affect early as well as late stages of secretion. Cloning and sequencing of the SEC3 gene revealed that it is identical to profilin synthetic lethal 1 (PSL1). The SEC3 gene is not essential because cells depleted of Sec3p are viable although slow growing and temperature sensitive. All of the sec3 alleles genetically interact with a profilin mutation, pfy1-111. The SEC3 gene in high copy suppresses pfy1-111 and sec5-24 and causes synthetic growth defects with ypt1, sec8-9, sec10-2, and sec15-1. Actin structure is only perturbed in conditions of chronic loss of Sec3p function, implying that Sec3p does not directly regulate actin. All alleles of sec3 cause bud site selection defects in homozygous diploids, as do sec4-8 and sec9-4. This suggests that SEC gene products are involved in determining the bud site and is consistent with a role for Sec3p in determining the correct site of exocytosis.

Synthetic lethality screen identifies a novel yeast myosin I gene (MYO5): myosin I proteins are required for polarization of the actin cytoskeleton

The organization of the actin cytoskeleton plays a critical role in cell physiology in motile and nonmotile organisms. Nonetheless, the function of the actin based motor molecules, members of the myosin superfamily, is not well understood. Deletion of MYO3, a yeast gene encoding a "classic" myosin I, has no detectable phenotype. We used a synthetic lethality screen to uncover genes whose functions might overlap with those of MYO3 and identified a second yeast myosin 1 gene, MYO5. MYO5 shows 86 and 62% identity to MYO3 across the motor and non-motor regions. Both genes contain an amino terminal motor domain, a neck region containing two IQ motifs, and a tail domain consisting of a positively charged region, a proline-rich region containing sequences implicated in ATP-insensitive actin binding, and an SH3 domain. Although myo5 deletion mutants have no detectable phenotype, yeast strains deleted for both MYO3 and MYO5 have severe defects in growth and actin cytoskeletal organization. Double deletion mutants also display phenotypes associated with actin disorganization including accumulation of intracellular membranes and vesicles, cell rounding, random bud site selection, sensitivity to high osmotic strength, and low pH as well as defects in chitin and cell wall deposition, invertase secretion, and fluid phase endocytosis. Indirect immunofluorescence studies using epitope-tagged Myo5p indicate that Myo5p is localized at actin patches. These results indicate that MYO3 and MYO5 encode classical myosin I proteins with overlapping functions and suggest a role for Myo3p and Myo5p in organization of the actin cytoskeleton of Saccharomyces cerevisiae.

Method for immobilizing microbial cells on gel surface for dynamic AFM studies

The processes of cell growth and budding of the yeast cells Saccharomyces cerevisiae, which were gently immobilized on 3% agar and submerged in culture medium, were successfully imaged with an atomic force microscope for 6-7 h. Similar experiments on chemically fixed cells did not detect any appreciable change in their appearance except in a few scannings at the very beginning, indicating that the dissolution of agar and/or scraping of its surface by the scanning tip, if any, did not significantly interfere with the images taken thereafter. The increment in the height of many of the untreated cells, accompanied by their lateral enlargement, was taken as an indication of successful imaging of the growth process of yeast cells, together with an image of a growing daughter cell attached to its mother cell.

Parallel secretory pathways to the cell surface in yeast

Saccharomyces cerevisiae mutants that have a post-Golgi block in the exocytic pathway accumulate 100-nm vesicles carrying secretory enzymes as well as plasma membrane and cell-wall components. We have separated the vesicle markers into two groups by equilibrium isodensity centrifugation. The major population of vesicles contains Bg12p, an endoglucanase destined to be a cell-wall component, as well as Pma1p, the major plasma membrane ATPase. In addition, Snc1p, a synaptobrevin homologue, copurifies with these vesicles. Another vesicle population contains the periplasmic enzymes invertase and acid phosphatase. Both vesicle populations also contain exoglucanase activity; the major exoglucanase normally secreted from the cell, encoded by EXG1, is carried in the population containing periplasmic enzymes. Electron microscopy shows that both vesicle groups have an average diameter of 100 nm. The late secretory mutants sec1, sec4, and sec6 accumulate both vesicle populations, while neither is detected in wild-type cells, early sec mutants, or a sec13 sec6 double mutant. Moreover, a block in endocytosis does not prevent the accumulation of either vesicle species in an end4 sec6 double mutant, further indicating that both populations are of exocytic origin. The accumulation of two populations of late secretory vesicles indicates the existence of two parallel routes from the Golgi to the plasma membrane.

The protein kinase C-activated MAP kinase pathway of Saccharomyces cerevisiae mediates a novel aspect of the heat shock response

The PKC1 gene of budding yeast encodes a homolog of the alpha, beta, and gamma isoforms of mammalian PKC that is proposed to regulate a MAPK-activation pathway. Mutants in this pathway undergo cell lysis resulting from a deficiency in cell wall construction when they attempt to grow at elevated temperatures. We show that the PKC1-regulated pathway is important for induced thermotolerance and that the MPK1 protein kinase (the MAPK of this pathway) is strongly activated by mild heat shock. This activation is sustained during growth at high temperature and is dependent on the function of pathway components proposed to function upstream of MPK1, including PKC1. Expression of genes under the control of known heat shock-inducible promoter elements (HSEs and STREs) was not compromised in PKC1 pathway mutants, indicating that this pathway mediates a novel aspect of the yeast heat shock response. We propose that the heat-induced signal for pathway activation is generated in response to weakness in the cell wall created during growth under thermal stress, perhaps as a result of increased membrane fluidity. Evidence is presented that the mechanism by which the cell detects this weakness is by measuring stretch of the plasma membrane.

Actin cortical cytoskeleton and cell wall synthesis in regenerating protoplasts of the Saccharomyces cerevisiae actin mutant DBY 1693

The relationship between the actin cytoskeleton and cell wall synthesis was studied by light and electron microscopy in protoplasts of Saccharomyces cerevisiae DBY 1693 containing the act1-1 allele. Since protoplasting also disturbs the actin cytoskeleton, these mutant protoplasts had a double error in their actin cytoskeletons. In the period between the onset of wall synthesis and completion of the wall, protoplasts grown at the permissive temperature showed an even distribution of actin patches all over the surface on which a new cell wall was being synthesized. After wall completion, actin patches partially disappeared, but then re-appeared, accumulated in growth regions at the start of polarized growth. This was compared with the pattern of actin patches observed in intact temperature-sensitive actin mutant cells cultivated at the permissive temperature. Electron microscopy of freeze-etched replicas revealed finger-like invaginations of the plasma membrane in both the actin mutant cells and their protoplasts. These structures showed a very similar distribution to the actin patches detected by rhodamine phalloidin staining in the fluorescence microscope. A hypothesis is presented, explaining the role of actin patches/finger-like invaginations of the plasma membrane in the synthesis of beta-(1-->3)-D-glucan wall microfibrils in yeast cells.

Cell cycle control of morphogenesis in budding yeast

A detailed description of the cytoskeletal rearrangements that orchestrate bud formation is beginning to emerge from studies on yeast morphogenesis. In this review, we focus on recent advances in our understanding of how the timing of these rearrangements is controlled. Dramatic changes in cell polarity that occur in G1 (polarization to the bud site), G2 (depolarization within the bud), and mitosis (repolarization to the mother/bud neck) are triggered by changes in the kinase activity of Cdc28, the universal regulator of cell cycle progression. The hunt for Cdc28 morphogenesis substrates is on.

Identification of two cell cycle regulated genes affecting the beta 1,3-glucan content of cell walls in Saccharomyces cerevisiae

The Calcofluor white-hypersensitive mutants cwh52 and cwh53 are severely reduced in beta 1,3-glucan. CWH52 was equivalent to GAS1. CWH53 represented a new gene, located on the right arm of chromosome XII, and predicted to encode a 215 kDa protein with multiple transmembrane domains. The transcription of CWH53 was cell cycle-dependent and, similar to GAS1/CWH52, increased in late G1, indicating that the formation of beta-glucan is cell cycle-regulated. Further, in some mutant alleles of both gas1/cwh52 and cwh53 lethal concentrations of Calcofluor induced growth arrest at a specific phase of the cell cycle.

Morphogenesis in the yeast cell cycle: regulation by Cdc28 and cyclins

Analysis of cell cycle regulation in the budding yeast Saccharomyces cerevisiae has shown that a central regulatory protein kinase, Cdc28, undergoes changes in activity through the cell cycle by associating with distinct groups of cyclins that accumulate at different times. The various cyclin/Cdc28 complexes control different aspects of cell cycle progression, including the commitment step known as START and mitosis. We found that altering the activity of Cdc28 had profound effects on morphogenesis during the yeast cell cycle. Our results suggest that activation of Cdc28 by G1 cyclins (Cln1, Cln2, or Cln3) in unbudded G1 cells triggers polarization of the cortical actin cytoskeleton to a specialized pre-bud site at one end of the cell, while activation of Cdc28 by mitotic cyclins (Clb1 or Clb2) in budded G2 cells causes depolarization of the cortical actin cytoskeleton and secretory apparatus. Inactivation of Cdc28 following cyclin destruction in mitosis triggers redistribution of cortical actin structures to the neck region for cytokinesis. In the case of pre-bud site assembly following START, we found that the actin rearrangement could be triggered by Cln/Cdc28 activation in the absence of de novo protein synthesis, suggesting that the kinase may directly phosphorylate substrates (such as actin-binding proteins) that regulate actin distribution in cells.

Effects of null mutations and overexpression of capping protein on morphogenesis, actin distribution and polarized secretion in yeast

CAP1, the gene encoding the alpha subunit of Saccharomyces cerevisiae capping protein, was cloned using a probe prepared by PCR with primers based on the amino acid sequence of purified alpha subunit peptides. The sequence is similar to that of capping protein alpha subunits of other species but not to that of the S. cerevisiae capping protein beta subunit or any other protein. Null mutants of capping protein, prepared by deletion of the coding region of CAP1 and CAP2 separately or together, are viable and have a similar phenotype. Deletion of the gene for one subunit leads to a loss of protein for the other subunit. The null mutant has a severe deficit of actin cables and an increased number of actin spots in the mother. Cells are round and relatively large. These features are heterogeneous within a population of cells and vary with genetic background. Overexpression of CAP1 and CAP2 also causes loss of actin cables and cell enlargement, as well as the additional traits of aberrant morphogenesis and cell wall thickening. Capping protein null strains and overexpression strains exhibited normal polarized secretion during bud growth as demonstrated by labeling with fluoresceinated Con A. Projection formation and chitin deposition in response to mating pheromone, mating efficiency, and bud site selection were also normal in capping protein null strains. In addition, bulk secretion of invertase was unimpaired. These data indicate that actin cables are not required for polarized secretion in S. cerevisiae.

Cyclic variations in the permeability of the cell wall of Saccharomyces cerevisiae

To study cell-cycle-related variations in wall permeability of Saccharomyces cerevisiae, two approaches were used. First, an asynchronous culture was fractionated by centrifugal elutriation into subpopulations containing cells of increasing size. The subpopulations represented different stages of the cell cycle as judged by light microscopy. Cell wall porosity increased when these subpopulations became enriched with budded cells. Secondly, synchronous cultures were obtained by releasing MATa cells from alpha-factor induced G1-arrest. These cultures grew synchronously for at least two generations. The cell wall porosity increased sharply in these cultures, shortly before buds became visible and was maximal during the initial stages of bud growth. It decreased in cells which had completed nuclear migration and before abscission of the bud had occurred. The porosity reached its lowest value during abscission and in unbudded cells. We examined the incorporation of mannoproteins into the wall during the cell cycle. SDS-extractable mannoproteins were incorporated continuously. However, the incorporation of glucanase-extractable mannoproteins, which are known to affect cell wall porosity, showed cyclic oscillations and reached its maximum after nuclear migration. This coincided with a rapid decrease in cell wall porosity, indicating that glucanase-extractable mannoproteins might contribute to this decrease.

An electron microscopy study of wall expansion during Candida albicans yeast and mycelial growth using concanavalin A-ferritin labelling of mannoproteins

Depending upon growth temperature, Candida albicans can exhibit two different morphologies, a budding yeast or a mycelium. By studying the distribution of concanavalin A-ferritin particles on the cell wall surface during bud and germ tube formation, we have elucidated the way cell wall extension occurs. Both processes initially require the localized lysis of the wall in order to allow the incorporation of the newly synthesized material. Later on, the cell wall behaves as an elastic structure, allowing extension by an intussusception process and, as a consequence, cell growth.

To shape a cell: an inquiry into the causes of morphogenesis of microorganisms

We recognize organisms first and foremost by their forms, but how they grow and shape themselves still largely passes understanding. The objective of this article is to survey what has been learned of morphogenesis of walled eucaryotic microorganisms as a set of problems in cellular heredity, biochemistry, physiology, and organization. Despite the diversity of microbial forms and habits, some common principles can be discerned. (i) That the form of each organism represents the expression of a genetic program is almost universally taken for granted. However, reflection on the findings with morphologically aberrant mutants suggests that the metaphor of a genetic program is misleading. Cellular form is generated by a web of interacting chemical and physical processes, whose every strand is woven of multiple gene products. The relationship between genes and form is indirect and cumulative; therefore, morphogenesis must be addressed as a problem not of molecular genetics but of cellular physiology. (ii) The shape of walled cells is determined by the manner in which the wall is laid down during growth and development. Turgor pressure commonly, perhaps always, supplies the driving force for surface enlargement. Cells yield to this scalar force by localized, controlled wall synthesis; their forms represent variations on the theme of local compliance with global force. (iii) Growth and division in bacteria display most immediately the interplay of hydrostatic pressure, localized wall synthesis, and structural constraints. Koch's surface stress theory provides a comprehensive and quantitative framework for understanding bacterial shapes. (iv) In the larger and more versatile eucaryotic cells, expansion is mediated by the secretion of vesicles. Secretion and ancillary processes, such as cytoplasmic transport, are spatially organized on the micrometer scale. The diversity of vectorial physiology and of the forms it generates is illustrated by examples: apical growth of fungal hyphae, bud formation in yeasts, germination of fucoid zygotes, and development of cells of Nitella, Closterium, and other unicellular algae. (v) Unicellular organisms, no less than embryos, have a remarkable capacity to impose spatial order upon themselves with or without the help of directional cues. Self-organization is reviewed here from two perspectives: the theoretical exploration of morphogens, gradients, and fields, and experimental study of polarization in Fucus cells, extension of hyphal tips, and pattern formation in ciliates. Here is the heart of the matter, yet self-organization remains nearly as mysterious as it was a century ago, a subject in search of a paradigm.

Isolation of nucleotide activated amino acid and peptide precursors of the pseudomurein of Methanobacterium thermoautotrophicum

The following putative precursors of the pseudomurein were isolated from trichloroacetic acid extracts of Methanobacterium thermoautotrophicum: a uridine diphosphate activated derivative of glutamic acid and the uridine diphosphate activated peptides (see text). The activated glutamic acid residue and the three activated pepetides lack the glycan components N-acetylglucosamine and N-acetyltalosaminuronic acid present in the intact pseudomurein. In this case uridine diphosphate should be directly linked to the amino group of a glutamic acid residue, which represents a new mode of amino acid and peptide activation.

Antigenic differences between mannoproteins of germ tubes and blastospores of Candida albicans

To determine the nature of germ tube-specific antigens of Candida albicans, procedures for intrinsically labeling cell wall antigens metabolically were developed. Blastospores or germ tubes labeled either in their proteins with L-[35S]methionine or in mannose-containing carbohydrates with D[2-3H]mannose contained surface components similar to those found previously with 125I-labeled organisms. Germ tube-specific determinants, were found on a 200-kilodalton protein in digests from germ tubes, whereas a component of similar molecular size in blastospore extracts reacted weakly or not at all with germ tube-specific antibody. In addition, a glycan fraction prepared from germ tubes reacted with the unadsorbed anti-C. albicans polyvalent antibody but not with the germ tube-specific antibody, suggesting that the germ tube-specific determinants are not carbohydrates.

Morphogenesis in Candida albicans

This review will survey environmental controls on the morphology of Candida albicans, describe the cellular and ultrastructural events associated with morphological transitions in this fungus, and attempt to relate biochemical phenomena that have been reported to be associated with dimorphic change to C. albicans cell biology. The synthesis of the cell wall of C. albicans and its control remain largely undiscovered, but it is clear that the cell wall is the principal component involved in shape determination. Possible models for C. albicans dimorphism will be critically reviewed.

Structural rearrangements of tubulin and actin during the cell cycle of the yeast Saccharomyces

The distribution of actin and tubulin during the cell cycle of the budding yeast Saccharomyces was mapped by immunofluorescence using fixed cells from which the walls had been removed by digestion. The intranuclear mitotic spindle was shown clearly by staining with a monoclonal antitubulin; the presence of extensive bundles of cytoplasmic microtubules is reported. In cells containing short spindles still entirely within the mother cells, one of the bundles of cytoplasmic microtubules nearly always extended to (or into) the bud. Two independent reagents (anti-yeast actin and fluorescent phalloidin) revealed an unusual distribution of actin: it was present as a set of cortical dots or patches and also as distinct fibers that were presumably bundles of actin filaments. Double labeling showed that at no stage in the cell cycle do the distributions of actin and tubulin coincide for any significant length, and, in particular, that the mitotic spindle did not stain detectably for actin. However, both microtubule and actin staining patterns change in a characteristic way during the cell cycle. In particular, the actin dots clustered in rings about the bases of very small buds and at the sites on unbudded cells at which bud emergence was apparently imminent. Later in the budding cycle, the actin dots were present largely in the buds and, in many strains, primarily at the tips of these buds. At about the time of cytokinesis the actin dots clustered in the neck region between the separating cells. These aspects of actin distribution suggest that it may have a role in the localized deposition of new cell wall material.

Metabolism of Saccharomyces cerevisiae envelope mannoproteins

By pulse and chase labeling experiments, two independent mannoprotein pools have been found associated with the Saccharomyces cerevisiae envelope. One of them probably corresponds to mannoproteins localized in the periplasmic space. These molecules showed a high turnover rate at 28 degrees C. The second pool is formed by intrinsic wall mannoproteins which are apparently stable for long periods of time, after a small initial turnover. These results suggest that at least part of the mannoproteins initially found in the periplasmic space may move into the wall. The time lag between the addition of the radioactive precursors and their incorporation in the cell envelope (20-30 min for amino acids and about 10 min for carbohydrate) indicates that protein formation and carbohydrate incorporation take place in succession. Moreover, bulk glycosylation of mannoproteins seems to occur close in time to the moment of secretion into the periplasmic space.

Roles of the CDC24 gene product in cellular morphogenesis during the Saccharomyces cerevisiae cell cycle

Temperature-sensitive yeast mutants defective in gene CDC24 continued to grow (i.e., increase in cell mass and cell volume) at restrictive temperature (36 degrees C) but were unable to form buds. Staining with the fluorescent dye Calcofluor showed that the mutants were also unable to form normal bud scars (the discrete chitin rings formed in the cell wall at budding sites) at 36 degrees C; instead, large amounts of chitin were deposited randomly over the surfaces of the growing unbudded cells. Labeling of cell-wall mannan with fluorescein isothiocyanate-conjugated concanavalin A suggested that mannan incorporation was also delocalized in mutant cells grown at 36 degrees C. Although the mutants have well-defined execution points just before bud emergence, inactivation of the CDC24 gene product in budded cells led both to selective growth of mother cells rather than of buds and to delocalized chitin deposition, indicating that the CDC24 gene product functions in the normal localization of growth in budded as well as in unbudded cells. Growth of the mutant strains at temperatures less than 36 degrees C revealed allele-specific differences in behavior. Two strains produced buds of abnormal shape during growth at 33 degrees C. Moreover, these same strains displayed abnormal localization of budding sites when growth at 24 degrees C (the normal permissive temperature for the mutants); in each case, the abnormal pattern of budding sites segregated with the temperature sensitivity in crosses. Thus, the CDC24 gene product seems to be involved in selection of the budding site, formation of the chitin ring at that site, the subsequent localization of new cell wall growth to the budding site and the growing bud, and the balance between tip growth and uniform growth of the bud that leads to the normal cell shape.