Regulation of G2/M Progression by the STE Mitogen-activated Protein Kinase Pathway in Budding Yeast Filamentous Growth

Abstract

Inoculation of diploid budding yeast onto nitrogen-poor agar media stimulates a MAPK pathway to promote filamentous growth. Characteristics of filamentous cells include a specific pattern of gene expression, elongated cell shape, polar budding pattern, persistent attachment to the mother cell, and a distinct cell cycle characterized by cell size control at G2/M. Although a requirement for MAPK signaling in filamentous gene expression is well established, the role of this pathway in the regulation of morphogenesis and the cell cycle remains obscure. We find that ectopic activation of the MAPK signal pathway induces a cell cycle shift to G2/M coordinately with other changes characteristic of filamentous growth. These effects are abrogated by overexpression of the yeast mitotic cyclins Clb1 and Clb2. In turn, yeast deficient for Clb2 or carrying cdc28-1N, an allele of CDK defective for mitotic functions, display enhanced filamentous differentiation and supersensitivity to the MAPK signal. Importantly, activation of Swe1-mediated inhibitory phosphorylation of Thr-18 and/or Tyr-19 of Cdc28 is not required for the MAPK pathway to affect the G2/M delay. Mutants expressing a nonphosphorylatable mutant Cdc28 or deficient for Swe1 exhibit low-nitrogen-dependent filamentous growth and are further induced by an ectopic MAPK signal. We infer that the MAPK pathway promotes filamentous growth by a novel mechanism that inhibits mitotic cyclin/CDK complexes and thereby modulates cell shape, budding pattern, and cell-cell connections.

Hyphal Elongation Is Regulated Independently of Cell Cycle inCandida albicans

“…Several recent publications on the relationship between pseudohyphal growth and Cdc28 activity in S. cerevisiae are supportive of this view (Ahn et al, 1999;Edgington et al, 1999;Loeb et al, 1999a). However, several lines of evidence in this study suggest that the model does not apply to the elongation of true hyphae in C. albicans.…”

“…They further hypothesized that cell elongation during pseudohyphal growth in S. cerevisiae might be caused by a delay in the apical-isotropic switch (Lew and Reed, 1993, 1995). In agreement with this, a grr1 mutant, which has stable G 1 cyclins, has enhanced pseudohyphal growth (Barral et al, 1995;Blacketer et al, 1995), as do strains mutated for B-type cyclins (Lew and Reed, 1993;Ahn et al, 1999). Furthermore, pseudohyphal S. cerevisiae cells exhibit symmetric cell division and have a longer G 2 phase than do yeast-form cells (Kron et al, 1994).…”

“…The percentages of mitotic cells were identical (Figure 2). It has been reported that S. cerevisiae pseudohyphal cells have a 12-15% longer G 2 phase than that of yeast-form cells (Ahn et al, 1999). Considering that hyphal elongation in C. albicans is much more dramatic than that of pseudohyphae in S. cerevisiae, the statistically significant 5% change in S/G 2 length is unlikely to account for the majority of apical growth observed in hyphal cells.…”

Modular organization of cellular networks

“…The Snf proteins control the carbon-source dependence of filamentation (24). Cdc28-associated proteins are involved in the altered cell-cycle progression of filamentousform cells (22,25). The a1͞␣2 cluster includes transcriptional regulators of cell-type specific (haploid versus diploid) development (21,23).…”

A Profile of Differentially Abundant Proteins at the Yeast Cell Periphery during Pseudohyphal Growth

“…During pseudohyphal growth, yeast cells delay in G 2 /M, resulting in an extended period of apically polarized growth and an elongated cell morphology (5)(6)(7). The yeast cells also exhibit an altered pattern of budding in which daughter cells emerge from mother cells predominantly opposite the birth end, as opposed to the bipolar pattern of bud emergence observed in diploid cells under conditions of vegetative growth (8).…”