567 In response to nitrogen limitation, diploid cells of the yeast Saccharomyces cerevisiae undergo a dimorphic transition to filamentous pseudohyphal growth. At least two signaling pathways regulate filamentation. One involves components of the MAP kinase cascade that also regulates mating of haploid cells. The second involves a nutrient-sensing G-protein- coupled receptor that signals via an unusual G α protein, cAMP and protein kinase A. Recent studies reveal crosstalk between these pathways during pseudohyphal growth. Related MAP kinase and cAMP pathways regulate filamentation and virulence of human and plant fungal pathogens, and represent novel targets for antifungal drug design. Addresses Departments of Genetics, Pharmacology and Cancer Biology, Microbiology, and Medicine, Howard Hughes Medical Institute, Duke University Medical Center, Durham, NC 27710, USA *322 CARL Building, Box 3546, Research Drive, Duke University Medical Center, Durham, NC 27710, USA; e-mail: heitm001@duke.edu Current Opinion in Microbiology 2000, 3:567–572 1369-5274/00/$ — see front matter © 2000 Elsevier Science Ltd. All rights reserved. Abbreviations GPCR G-protein-coupled receptor MAP mitogen-activated protein PKA protein kinase A Plc1 phospholipase C Introduction The yeast Saccharomyces cerevisiae and many human and plant fungal pathogens adopt either yeast or filamentous growth forms in response to different environmental sig- nals. The ability to undergo this dimorphic transition is an important determinant of virulence in fungal pathogens. Recent studies on a filamentous growth form known as pseudohyphal differentiation in S. cerevisiae have provided substantial insights into the molecular mechanisms governing filamentous growth that are conserved in many other fungi. Under conditions of nitrogen limitation, diploid strains of the yeast S. cerevisiae switch from growth as individual oval cells to filamentous growth as chains of elongated cells [1]. Filamentous growth represents a dramatic change in growth pattern, during which the cells elongate and switch to a unipolar budding pattern, the mother and daughter cells remain physically attached, and the cells invade the growth substrate (Figure 1) [1,2]. Here we review the sig- naling pathways that regulate yeast pseudohyphal differentiation, emphasizing the progress that has been made during the past two years. A MAP kinase pathway regulates yeast filamentous growth Two signaling pathways that regulate yeast filamentous growth have been defined (Figure 2). The first involves the mitogen-activated protein (MAP) kinase pathway that also functions during mating between haploid cells of opposite mating types [3–6], possibly as a result of co-evo- lution of mating and filamentous growth. The components of this cascade required for filamentous growth include the Ste20, Ste11, Ste7 and Kss1 kinases. This pathway inacti- vates the repressors Dig1 and Dig2, allowing the transcription factors Ste12 and Tec1 to form heterodimers that regulate expression of Tec1 itself and additional targets, such as Flo11 [7–11]. Flo11 is a glycosylphos- phatidylinositol-anchored cell-surface protein required for calcium-dependent cell–cell adhesion (also known as floc- culation) [9,12]. The upstream components that activate the MAP kinase pathway during pseudohyphal differentia- tion include Ras2, Cdc42 and the 14-3-3 proteins Bmh1 and Bmh2 [13–15], which may be activated by the Sho1 osmosensing receptor [16,17]. In contrast, the upstream components of the mating pathway, including the pheromones and their receptors Ste2 or Ste3, the G-protein βγ subunit Ste4–Ste18 and the scaffold protein Ste5, are not required for diploid filamentous growth [3]. Signal transduction cascades regulating pseudohyphal differentiation of Saccharomyces cerevisiae Xuewen Pan, Toshiaki Harashima and Joseph Heitman* Figure 1 Diploid cells of the yeast Saccharomyces cerevisiae undergo pseudohyphal differentiation in response to nitrogen limitation. In response to nitrogen-limiting conditions, diploid cells of S. cerevisiae change their growth pattern: the cells elongate and switch from bipolar to unipolar budding, the mother and daughter cells remain physically attached, and the cells invade the growth substrate. As a result, filamentous pseudohyphal colonies are formed. Fermentable carbon source Nitrogen deprivation Bipolar budding Oval cells 1 2 3 3 4 4 Unipolar budding Elongated cells Mother–daughter cell adhesion Agar invasion 1 2 3 3 4 4 Current Opinion in Microbiology