Development of cell polarity in budding yeast

Functional genomics in the study of yeast cell polarity: moving in the right direction

The budding yeast Saccharomyces cerevisiae has been used extensively for the study of cell polarity, owing to both its experimental tractability and the high conservation of cell polarity and other basic biological processes among eukaryotes. The budding yeast has also served as a pioneer model organism for virtually all genome-scale approaches, including functional genomics, which aims to define gene function and biological pathways systematically through the analysis of high-throughput experimental data. Here, we outline the contributions of functional genomics and high-throughput methodologies to the study of cell polarity in the budding yeast. We integrate data from published genetic screens that use a variety of functional genomics approaches to query different aspects of polarity. Our integrated dataset is enriched for polarity processes, as well as some processes that are not intrinsically linked to cell polarity, and may provide new areas for future study.

Functional characterization of Aspergillus nidulans ypkA, a homologue of the mammalian kinase SGK

The serum- and glucocorticoid-regulated protein kinase (SGK) is an AGC kinase involved in signal cascades regulated by glucocorticoid hormones and serum in mammals. The Saccharomyces cerevisiae ypk1 and ypk2 genes were identified as SGK homologues and Ypk1 was shown to regulate the balance of sphingolipids between the inner and outer plasma membrane. This investigation characterized the Aspergillus nidulans YPK1 homologue, YpkA, representing the first filamentous fungal YPK1 homologue. Two conditional mutant strains were constructed by replacing the endogenous ypk1 promoter with two different regulatable promoters, alcA (from the alcohol dehydrogenase gene) and niiA (from the nitrate reductase gene). Both constructs confirmed that ypkA was an essential gene in A. nidulans. Repression of ypkA caused decreased radial growth, a delay in conidial germination, deficient polar axis establishment, intense branching during late stages of growth, a lack of asexual spores, and a terminal phenotype. Membrane lipid polarization, endocytosis, eisosomes and vacuolar distribution were also affected by ypkA repression, suggesting that YpkA plays a role in hyphal morphogenesis via coordinating the delivery of cell membrane and wall constituents to the hyphal apex. The A. nidulans Pkh1 homologue pkhA was also shown to be an essential gene, and preliminary genetic analysis suggested that the ypkA gene is not directly downstream of pkhA or epistatic to pkhA, rather, ypkA and pkhA are genetically independent or in parallel. BarA is a homologue of the yeast Lag1 acyl-CoA-dependent ceramide synthase, which catalyzes the condensation of phytosphingosine with a fatty acyl-CoA to form phytoceramide. When barA was absent, ypkA repression was lethal to the cell. Therefore, there appears to be a genetic interaction between ypkA, barA, and the sphingolipid synthesis. Transcriptional profiling of ypkA overexpression and down-regulation revealed several putative YpkA targets associated with the observed phenotypes.

A macroarray expression analysis of novel cDNAs vital for growth initiation and primary metabolism during development of Heterobasidion parviporum conidiospores

The pathogen Heterobasidion parviporum causes significant losses to forest industries in Europe and North America. The fungus is spread by basidiospores on fresh stumps where it differentiates into a specialized infection hyphae to colonize its host. This differentiation is driven by recognition and its strategic success lies in its ability to do this rapidly and efficiently. To investigate gene expression pattern during the spore germination stages, mRNA of germinated and ungerminated conidiospores of H. parviporum harvested at distinct developmental time points (18, 36, 72 and 120 h) corresponding to periods of isotropic/germ tube emergence, polarized apical, early and late mycelial lateral branching growth stages was hybridized to macroarrays containing 338 cDNAs from H. parviporum. The results of the statistical analysis identified a total of 24, 39, 38 and 30 genes that were differentially upregulated at 18, 36, 72 and 120 h, respectively, relative to time 0. The number of the downregulated genes was 4, 6, 8 and 13 genes respectively. During isotropic, polarized and mycelial growth stages, majority of the differentially expressed genes belonged to functional categories metabolism (21-32%) and protein formation (21-30%). Real-time polymerase chain reaction (real-time-PCR) data essentially confirmed the macroarray analyses. The real-time-PCR result showed that transcript levels of genes involved in glucose metabolism (phosphoglucomutase), amino acid metabolism (arginase, delta-1-pyrroline-5-carboxylate reductase, sulfur metabolism-negative regulator, imidazoleglycerol phosphate dehydratase) and protein synthesis (40S ribosomal protein S15) were significantly increased during polarized growth (36 h) stage but decreased at early and late stages of mycelial growth (72-120 h). An understanding of the various molecular and physiological processes during the development of H. parviporum spores is an important step towards the goal of identifying novel antifungal strategies.

In vitro fusion between Saccharomyces cerevisiae secretory vesicles and cytoplasmic-side-out plasma membrane vesicles

The final step in the secretory pathway, which is the fusion event between secretory vesicles and the plasma membrane, was reconstructed using highly purified secretory vesicles and cytoplasmic-side-out plasma membrane vesicles from the yeast Saccharomyces cerevisiae. Both organelle preparations were obtained from a sec 6-4 temperature-sensitive mutant. Fusion was monitored by means of a fluorescence assay based on the dequenching of the lipophilic fluorescent probe octadecylrhodamine B-chloride (R18). The probe was incorporated into the membrane of secretory vesicles, and it diluted in unlabelled cytoplasmic-side-out plasma membrane vesicles as the fusion process took place. The obtained experimental dequenching curves were found by mathematical analysis to consist of two independent but simultaneous processes. Whereas one of them reflected the fusion process between both vesicle populations as confirmed by its dependence on the assay conditions, the other represented a non-specific transfer of the probe. The fusion process may now be examined in detail using the preparation, validation and analytical methods developed in this study.

Yeast 1,3-beta-glucan synthase activity is inhibited by phytosphingosine localized to the endoplasmic reticulum

1,3-beta-D-Glucan, a major filamentous component of the cell wall in the budding yeast Saccharomyces cerevisiae, is synthesized by 1,3-beta-glucan synthase (GS). Although a yeast gene whose product is required for GS activity in vitro, GNS1, was isolated and characterized, its role in GS function has remained unknown. In the current study we show that Deltagns1 cells accumulate a non-competitive and non-proteinous inhibitor(s) in the membrane fraction. Investigations of inhibitory activity on GS revealed that the inhibitor(s) is mainly present in the sphingolipid fraction. It is shown that Deltagns1 cells contain phytosphingosine (PHS), an intermediate in the sphingolipid biosynthesis, 30-fold more than wild-type cells do. The membrane fraction isolated from Deltasur2 cells contains an increased amount of dihydrosphingosine (DHS) and also exhibits reduced GS activity. Among constituents of the sphingolipid fraction, PHS and DHS show striking inhibition in a non-competitive manner. The intracellular level of DHS is much lower than that of PHS in wild-type cells, suggesting that PHS is the primary inhibitor of GS in vivo. The localization of PHS to the endoplasmic reticulum in wild-type cells coincides with that of the inhibitor(s) in Deltagns1 cells. Taken together, our results indicate that PHS is a potent inhibitor of yeast GS in vivo.

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.

Identification and characterization of genes required for hyphal morphogenesis in the filamentous fungus Aspergillus nidulans

In the filamentous fungus Aspergillus nidulans, germination of an asexual conidiospore results in the formation of a hyphal cell. A key feature of spore germination is the switch from isotropic spore expansion to polarized apical growth. Here, temperature-sensitive mutations are used to characterize the roles of five genes (sepA, hypA, podB-podD) in the establishment and maintenance of hyphal polarity. Evidence that suggests that the hypA, podB, and sepA genes are required for multiple aspects of hyphal morphogenesis is presented. Notably, podB and sepA are needed for organization of the cytoskeleton at sites of polarized growth. In contrast, podC and podD encode proteins that appear to be specifically required for the establishment of hyphal polarity during spore germination. The role of sepA and the pod genes in controlling the spatial pattern of polarized morphogenesis in germinating spores is also described. Results obtained from these experiments indicate that the normal pattern of germ-tube emergence is dependent upon the integrity of the actin cytoskeleton.

Cdc42: An essential Rho-type GTPase controlling eukaryotic cell polarity

Cdc42p is an essential GTPase that belongs to the Rho/Rac subfamily of Ras-like GTPases. These proteins act as molecular switches by responding to exogenous and/or endogenous signals and relaying those signals to activate downstream components of a biological pathway. The 11 current members of the Cdc42p family display between 75 and 100% amino acid identity and are functional as well as structural homologs. Cdc42p transduces signals to the actin cytoskeleton to initiate and maintain polarized gorwth and to mitogen-activated protein morphogenesis. In the budding yeast Saccharomyces cerevisiae, Cdc42p plays an important role in multiple actin-dependent morphogenetic events such as bud emergence, mating-projection formation, and pseudohyphal growth. In mammalian cells, Cdc42p regulates a variety of actin-dependent events and induces the JNK/SAPK protein kinase cascade, which leads to the activation of transcription factors within the nucleus. Cdc42p mediates these processes through interactions with a myriad of downstream effectors, whose number and regulation we are just starting to understand. In addition, Cdc42p has been implicated in a number of human diseases through interactions with its regulators and downstream effectors. While much is known about Cdc42p structure and functional interactions, little is known about the mechanism(s) by which it transduces signals within the cell. Future research should focus on this question as well as on the detailed analysis of the interactions of Cdc42p with its regulators and downstream effectors.

Cell polarity and morphogenesis in budding yeast

Eukaryotic cells respond to intracellular and extracellular cues to direct asymmetric cell growth and division. The yeast Saccharomyces cerevisiae undergoes polarized growth at several times during budding and mating and is a useful model organism for studying asymmetric growth and division. In recent years, many regulatory and cytoskeletal components important for directing and executing growth have been identified, and molecular mechanisms have been elucidated in yeast. Key signaling pathways that regulate polarization during the cell cycle and mating response have been described. Since many of the components important for polarized cell growth are conserved in other organisms, the basic mechanisms mediating polarized cell growth are likely to be universal among eukaryotes.

New perspectives on mechanisms involved in generating epithelial cell polarity

Polarized epithelial cells form barriers that separate biological compartments and regulate homeostasis by controlling ion and solute transport between those compartments. Receptors, ion transporters and channels, signal transduction proteins, and cytoskeletal proteins are organized into functionally and structurally distinct domains of the cell surface, termed apical and basolateral, that face these different compartments. This review is about mechanisms involved in the establishment and maintenance of cell polarity. Previous reports and reviews have adopted a Golgi-centric view of how epithelial cell polarity is established, in which the sorting of apical and basolateral membrane proteins in the Golgi complex is a specialized process in polarized cells, and the generation of cell surface polarity is a direct consequence of this process. Here, we argue that events at the cell surface are fundamental to the generation of cell polarity. We propose that the establishment of structural asymmetry in the plasma membrane is the first, critical event, and subsequently, this asymmetry is reinforced and maintained by delivery of proteins that were constitutively sorted in the Golgi. We propose a hierarchy of stages for establishing cell polarity.

Polarity and division site specification in yeast

A significant component of polarization in budding yeast involves the regulated restructuring of the actin cytoskeleton in response to defined cellular signals. Recent evidence suggests that such cytoskeletal organization arises through the action of large protein complexes that form in response to signals from small GTP-binding proteins, such as Cdc42, Rho, and Ras. These actin-organizing complexes may be fairly diverse, but generally consist of one or more central scaffold proteins, such as those of the formin class, that bind to signaling molecules and recruit actin-binding proteins to bring about desired polarizing events.

Regulation of SNARE complex assembly by an N-terminal domain of the t-SNARE Sso1p

The fusion of intracellular transport vesicles with their target membranes requires the assembly of SNARE proteins anchored in the apposed membranes. Here we use recombinant cytoplasmic domains of the yeast SNAREs involved in Golgi to plasma membrane trafficking to examine this assembly process in vitro. Binary complexes form between the target membrane SNAREs Sso1p and Sec9p; these binary complexes can subsequently bind to the vesicle SNARE Snc2p to form ternary complexes. Binary and ternary complex assembly are accompanied by large increases in alpha-helical structure, indicating that folding and complex formation are linked. Surprisingly, we find that binary complex formation is extremely slow, with a second-order rate constant of approximately 3 M(-1) s(-1). An N-terminal regulatory domain of Sso1p accounts for slow assembly, since in its absence complexes assemble 2,000-fold more rapidly. Once binary complexes form, ternary complex formation is rapid and is not affected by the presence of the regulatory domain. Our results imply that proteins that accelerate SNARE assembly in vivo act by relieving inhibition by this regulatory domain.

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.

Mating type switching in yeast controlled by asymmetric localization of ASH1 mRNA

Cell divisions that produce progeny differing in their patterns of gene expression are key to the development of multicellular organisms. In the budding yeast Saccharomyces cerevisiae, mother cells but not daughter cells can switch mating type because they selectively express the HO endonuclease gene. This asymmetry is due to the preferential accumulation of an unstable transcriptional repressor protein, Ash1p, in daughter cell nuclei. Here it is shown that ASH1 messenger RNA (mRNA) preferentially accumulates in daughter cells by a process that is dependent on actin and myosin. A cis-acting element in the 3'-untranslated region of ASH1 mRNA is sufficient to localize a chimeric RNA to daughter cells. These results suggest that localization of mRNA may have been an early property of the eukaryotic lineage.

Sec2p mediates nucleotide exchange on Sec4p and is involved in polarized delivery of post-Golgi vesicles

The small GTPase Sec4p is required for vesicular transport at the post-Golgi stage of yeast secretion. Here we present evidence that mutations in SEC2, itself an essential gene that acts at the same stage of the secretory pathway, cause Sec4p to mislocalize as a result of a random rather than a polarized accumulation of vesicles. Sec2p and Sec4p interact directly, with the nucleotide-free conformation of Sec4p being the preferred state for interaction with Sec2p. Sec2p functions as an exchange protein, catalyzing the dissociation of GDP from Sec4 and promoting the binding of GTP. We propose that Sec2p functions to couple the activation of Sec4p to the polarized delivery of vesicles to the site of exocytosis.