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Signaling molecules such as Cdc42 and mitogen-activated protein kinases (MAPKs) can function in multiple pathways in the same cell. Here, we propose one mechanism by which such factors may be directed to function in a particular pathway such that a specific response is elicited. Using genomic approaches, we identify a new component of the Cdc42-and MAPK-dependent signaling pathway that regulates filamentous growth (FG) in yeast. This factor, called Msb2, is a FG-pathway-specific factor that promotes differential activation of the MAPK for the FG pathway, Kss1. Msb2 is localized to polarized sites on the cell surface and interacts with Cdc42 and with the osmosensor for the high osmolarity glycerol response (HOG) pathway, Sho1. Msb2 is glycosylated and is a member of the mucin family, proteins that in mammalian cells promote disease resistance and contribute to metastasis in cancer cells. Remarkably, loss of the mucin domain of Msb2 causes hyperactivity of the FG pathway, demonstrating an inhibitory role for mucin domains in MAPK pathway activation. Taken together, our data suggest that Msb2 is a signaling mucin that interacts with general components, such as Cdc42 and Sho1, to promote their function in the FG pathway.[Keywords: Morphogenesis; cell polarity; signal transduction; pseudohyphal growth; specificity] Supplemental material is available at http://www.genesdev.org.

Haploid yeast invades solid agar in response to nutrient limitation. To decipher the cues that underlie invasion, we have developed a single cell invasive growth assay. Using this assay, as well as the traditional plate-washing assay, we show that invasive growth occurs in response to glucose depletion. In the absence of glucose (or other fermentable sugar), individual cells adopted a nonaxial budding pattern and elongated morphology within the first cell divisions, and invasion into the agar was observed in microcolonies containing as few as 10 cells. In support of this observation, we found that glucose suppressed the hyperinvasive growth morphology of STE11-4, pbs2, hsl7, and RAS2V19 mutations. In addition, removal of glucose from YPD medium caused constitutive invasion in wild-type cells. We tested glucose control proteins for a role in invasion and found that Snf1, a protein required for derepression of glucose-repressed genes, was required for invasive growth. The transcription factor Sip4, which interacts with Snf1 and is induced during the diauxic shift, had an inhibitory role on invasive growth, suggesting that multiple mechanisms are required for glucose depletion-dependent invasion. S everal yeast͞fungal species have the ability to adopt two growth forms, a vegetative (yeast) form and a pseudohyphal or filamentous form. In pathogenic organisms such as Candida albicans, this morphological transition is important for invasion of host tissue and therefore pathogenesis (1). In nonpathogenic organisms, the transition can occur upon nutrient limitation and has therefore been suggested to be a mechanism to permit foraging for nutrients. Insight into the signal transduction mechanisms that are necessary for the morphological change will contribute to an understanding of basic biological phenomena and of pathogenesis.The budding yeast Saccharomyces cerevisiae provides a genetic system to investigate the morphological transition from vegetative to filamentous development. In diploid yeast, loss of environmental fixed nitrogen causes the transition from vegetative growth, in which cells are round and bud in a bipolar fashion, to pseudohyphal growth, in which cells are elongated and bud in a unipolar budding pattern (2). In haploid yeast, nutrient limitation causes a similar developmental switch that allows cells to penetrate the surface of an agar medium in a process called invasive growth (see refs. 3-6 for reviews). For simplicity, we will use the term filamentous form to refer to both pseudohyphal and invasive growth, although there are differences between these two growth responses.At least two signal transduction pathways are required for filamentous growth. One pathway uses a number of components from the pheromone response pathway, notably the p21-activated kinase (PAK) kinase Ste20, the kinases Ste11 and Ste7 from the first two tiers of the mitogen-activated protein (MAP) kinase cascade, and the transcription factor Ste12 (7, 8), but has unique components as well-the MAP kinase Kss1 (9-11) and a second t...

Abstract. The Saccharomyces cerevisiae a-factor receptor (STE3) is subject to two modes of endocytosis: a constitutive process that occurs in the absence of ligand and a regulated process that is triggered by binding of ligand. Both processes result in delivery of the receptor to the vacuole for degradation. Receptor mutants deleted for part of the COOH-terminal cytoplasmic domain are disabled for constitutive, but not ligand-dependent internalization. Trans-acfing mutants that impair constitutive endocytosis have been isolated.One of these, renl-1, is blocked at a late step in the endocytic pathway, as receptor accumulates in a prevacuolar endosome-like compartment. REN1 is identical to VPS2, a gene required for delivery of newly synthesized vacuolar enzymes to the vacuole. Based on this identity, we suggest a model in which the transport pathways to the vacuole-the endocytic pathway and the vacuolar biogenesis pathway-merge at an intermediate endocytic compartment. As receptor also accumulates at the surface of renl cells, receptor may recycle from the putative endosome to the surface, or REN1 may also be required to carry out an early step in endocytosis.

Filamentous growth is a nutrient-regulated growth response that occurs in many fungal species. In pathogens, filamentous growth is critical for host-cell attachment, invasion into tissues, and virulence. The budding yeast Saccharomyces cerevisiae undergoes filamentous growth, which provides a genetically tractable system to study the molecular basis of the response. Filamentous growth is regulated by evolutionarily conserved signaling pathways. One of these pathways is a mitogen activated protein kinase (MAPK) pathway. A remarkable feature of the filamentous growth MAPK pathway is that it is composed of factors that also function in other pathways. An intriguing challenge therefore has been to understand how pathways that share components establish and maintain their identity. Other canonical signaling pathways-rat sarcoma/protein kinase A (RAS/PKA), sucrose nonfermentable (SNF), and target of rapamycin (TOR)-also regulate filamentous growth, which raises the question of how signals from multiple pathways become integrated into a coordinated response. Together, these pathways regulate cell differentiation to the filamentous type, which is characterized by changes in cell adhesion, cell polarity, and cell shape. How these changes are accomplished is also discussed. High-throughput genomics approaches have recently uncovered new connections to filamentous growth regulation. These connections suggest that filamentous growth is a more complex and globally regulated behavior than is currently appreciated, which may help to pave the way for future investigations into this eukaryotic cell differentiation behavior. TABLE OF CONTENTS Abstract 23Introduction 24