The CDC42 Homolog of the Dimorphic Fungus Penicillium marneffei Is Required for Correct Cell Polarization during Growth but Not Development

Abstract

The opportunistic human pathogenic fungus Penicillium marneffei is dimorphic and is thereby capable of growth either as filamentous multinucleate hyphae or as uninucleate yeast cells which divide by fission. The dimorphic switch is temperature dependent and requires regulated changes in morphology and cell shape. Cdc42p is a Rho family GTPase which in Saccharomyces cerevisiae is required for changes in polarized growth during mating and pseudohyphal development. Cdc42p homologs in higher organisms are also associated with changes in cell shape and polarity. We have cloned a highly conserved CDC42 homolog from P. marneffei named cflA. By the generation of dominant-negative and dominant-activated cflA transformants, we have shown that CflA initiates polarized growth and extension of the germ tube and subsequently maintains polarized growth in the vegetative mycelium. CflA is also required for polarization and determination of correct cell shape during yeast-like growth, and active CflA is required for the separation of yeast cells. However, correct cflA function is not required for dimorphic switching and does not appear to play a role during the generation of specialized structures during asexual development. In contrast, heterologous expression of cflA alleles in Aspergillus nidulans prevented conidiation.

Control of morphogenesis and actin localization by thePenicillium marneffei RAChomolog

“…The difference in the regulation of polarized growth during asexual development in A. nidulans and P. marneffei also extends to other Rho GTPases. A. nidulans transformants expressing mutant cflB alleles show hyperpolarization of conidiophores, whereas overexpression of the P. marneffei CDC42 homolog cflA in A. nidulans completely blocks conidiation (Boyce, 2001). In P. marneffei, expression of dominant-negative or -activated cflB alleles reduces polarized growth during conidiation but, when the dominant-negative or -activated cflA alleles are expressed, conidiation is not affected.…”

“…In P. marneffei, the Cdc42 and Rac proteins possess coordinate roles during development. Both CflA and CflB are required for vegetative growth at 25°C, although the roles appear to be distinct because the phenotypes are not identical (Boyce et al, 2001). CflA and CflB also possess distinct roles during development.…”

“…Correct CflA function is required for polarized growth of vegetative hyphae at 25°C and yeast cell morphogenesis at 37°C. By contrast, cflA was not involved in asexual development at 25°C and no other regulator of polarized growth during asexual development was known (Boyce et al, 2001). During the cloning of cflA, a second related gene was identified, designated cflB, which has been found to encode another Rho GTPase, with specific homology to the RAC class.…”

Regulation of polarised growth in fungi

“…Attempts to delete cflA were unsuccessful, suggesting that like yeasts, Cdc42p is essential for viability (Boyce et al, 2001). Its role was investigated by the expression of cflA D120A , which is equivalent to the S. cerevisiae CDC42 D118A dominant negative allele.…”

“…Thus, in A. nidulans ModA, the Cdc42 homologue, and RacA also share an essential overlapping function. A tentative picture that emerges is that in two ascomycetes (P. marneffei, A.nidulans) and a basidomycete (Ustilago maydis) the functions of Cdc42p in S. cerevisiae are divided between Cdc42p and Rac1p (Boyce et al, 2001;Boyce et al, 2003;Boyce et al, 2005;Mahlert et al, 2006;Virag et al, 2007).…”