Sok2 regulates yeast pseudohyphal differentiation via a transcription factor cascade that regulates cell-cell adhesion

The basic helix-loop-helix transcription factor Cph2 regulates hyphal development in Candida albicans partly via TEC1

Candida albicans undergoes a morphogenetic switch from budding yeast to hyphal growth form in response to a variety of stimuli and growth conditions. Multiple signaling pathways, including a Cph1-mediated mitogen-activated protein kinase pathway and an Efg1-mediated cyclic AMP/protein kinase A pathway, regulate the transition. Here we report the identification of a basic helix-loop-helix transcription factor of the Myc subfamily (Cph2) by its ability to promote pseudohyphal growth in Saccharomyces cerevisiae. Like sterol response element binding protein 1, Cph2 has a Tyr instead of a conserved Arg in the basic DNA binding region. Cph2 regulates hyphal development in C. albicans, as cph2/cph2 mutant strains show medium-specific impairment in hyphal development and in the induction of hypha-specific genes. However, many hypha-specific genes do not have potential Cph2 binding sites in their upstream regions. Interestingly, upstream sequences of all known hypha-specific genes are found to contain potential binding sites for Tec1, a regulator of hyphal development. Northern analysis shows that TEC1 transcription is highest in the medium in which cph2/cph2 displays a defect in hyphal development, and Cph2 is necessary for this transcriptional induction of TEC1. In vitro gel mobility shift experiments show that Cph2 directly binds to the two sterol regulatory element 1-like elements upstream of TEC1. Furthermore, the ectopic expression of TEC1 suppresses the defect of cph2/cph2 in hyphal development. Therefore, the function of Cph2 in hyphal transcription is mediated, in part, through Tec1. We further show that this function of Cph2 is independent of the Cph1- and Efg1-mediated pathways.

Overlapping and distinct roles of the duplicated yeast transcription factors Ace2p and Swi5p

The yeast transcription factors Ace2p and Swi5p regulate the expression of several target genes involved in mating type switching, exit from mitosis and cell wall function. We describe the analysis of 12 novel targets, some regulated by Ace2p or Swi5p alone and some by both. We show that Ace2p is the major regulator of four genes (CTS1, YHR143W, SCW11 and YER124C). Expression of all four is inhibited by Swi5p. Like Cts1p and Scw11p, the two new Ace2p targets are associated with cell wall metabolism. Yhr143p is localized to the cell wall, and deletion affects cell separation and enhances pseudohyphal growth. Deleting YER124C also affects cell separation and sensitivity to drugs targeted against the cell wall. Expression of PIR1, YPL158C and YNL046W is dependent on Swi5p alone. In contrast, expression of YBR158W, YNL078W and YOR264W is minimized when both ace2 and swi5 are disrupted. We propose that, although Ace2p and Swi5p co-operate to induce the expression of a subset of genes, some functional divergence has occurred. This results in a delay in the expression of those genes predominantly regulated by Ace2p, compared with those predominantly regulated by Swi5p.

Interaction of the E1A oncoprotein with Yak1p, a novel regulator of yeast pseudohyphal differentiation, and related mammalian kinases

The C-terminal portion of adenovirus E1A suppresses ras-induced metastasis and tumorigenicity in mammalian cells; however, little is known about the mechanisms by which this occurs. In the simple eukaryote Saccharomyces cerevisiae, Ras2p, the homolog of mammalian h-ras, regulates mitogen-activated protein kinase (MAPK) and cyclic AMP-dependent protein kinase A (cAMP/PKA) signaling pathways to control differentiation from the yeast form to the pseudohyphal form. When expressed in yeast, the C-terminal region of E1A induced pseudohyphal differentiation, and this was independent of both the MAPK and cAMP/PKA signaling pathways. Using the yeast two-hybrid system, we identified an interaction between the C-terminal region of E1A and Yak1p, a yeast dual-specificity serine/threonine protein kinase that functions as a negative regulator of growth. E1A also physically interacts with Dyrk1A and Dyrk1B, two mammalian homologs of Yak1p, and stimulates their kinase activity in vitro. We further demonstrate that Yak1p is required in yeast to mediate pseudohyphal differentiation induced by Ras2p-regulated signaling pathways. However, pseudohyphal differentiation induced by the C-terminal region of E1A is largely independent of Yak1p. These data suggest that mammalian Yak1p-related kinases may be targeted by the E1A oncogene to modulate cell growth.

Signal transduction cascades regulating pseudohyphal differentiation of Saccharomyces cerevisiae

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(alpha) 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.

Signal transduction cascades regulating fungal development and virulence

Cellular differentiation, mating, and filamentous growth are regulated in many fungi by environmental and nutritional signals. For example, in response to nitrogen limitation, diploid cells of the yeast Saccharomyces cerevisiae undergo a dimorphic transition to filamentous growth referred to as pseudohyphal differentiation. Yeast filamentous growth is regulated, in part, by two conserved signal transduction cascades: a mitogen-activated protein kinase cascade and a G-protein regulated cyclic AMP signaling pathway. Related signaling cascades play an analogous role in regulating mating and virulence in the plant fungal pathogen Ustilago maydis and the human fungal pathogens Cryptococcus neoformans and Candida albicans. We review here studies on the signaling cascades that regulate development of these and other fungi. This analysis illustrates both how the model yeast S. cerevisiae can serve as a paradigm for signaling in other organisms and also how studies in other fungi provide insights into conserved signaling pathways that operate in many divergent organisms.

Signal transduction cascades regulating fungal development and virulence

Cellular differentiation, mating, and filamentous growth are regulated in many fungi by environmental and nutritional signals. For example, in response to nitrogen limitation, diploid cells of the yeast Saccharomyces cerevisiae undergo a dimorphic transition to filamentous growth referred to as pseudohyphal differentiation. Yeast filamentous growth is regulated, in part, by two conserved signal transduction cascades: a mitogen-activated protein kinase cascade and a G-protein regulated cyclic AMP signaling pathway. Related signaling cascades play an analogous role in regulating mating and virulence in the plant fungal pathogen Ustilago maydis and the human fungal pathogens Cryptococcus neoformans and Candida albicans. We review here studies on the signaling cascades that regulate development of these and other fungi. This analysis illustrates both how the model yeast S. cerevisiae can serve as a paradigm for signaling in other organisms and also how studies in other fungi provide insights into conserved signaling pathways that operate in many divergent organisms.