Control of morphogenesis and actin localization by the Penicillium marneffei RAC homolog

Rac proteins control polarized growth in many organisms but the specific function of these proteins remains undefined. In this study, we describe the cloning and functional characterization of a RAC homolog, cflB, from the dimorphic fungus Penicillium marneffei. P. marneffei produces asexual spores on complex structures (conidiophores)and switches between hyphal and yeast growth. CflB colocalizes with actin at the tips of vegetative hyphal cells and at sites of cell division. Deletion of cflB results in cell division (septation) and growth defects in both vegetative hyphal and conidiophore cell types such that cells become depolarized, exhibit inappropriate septation and the actin cytoskeleton is severely disrupted. This data suggests that Rac proteins play a crucial role in actin dependent polarized growth and division. The CDC42 ortholog in P. marneffei, cflA, controls vegetative hyphal and yeast growth polarization but does not affect asexual development. By contrast, CflB affects cellular polarization during asexual development and hyphal growth but not during yeast growth. This shows that these two GTPases have both overlapping and distinct roles during growth and development. RAC orthologs are not found in less morphologically complex eukaryotes such as Saccharomyces cerevisiae, suggesting that RAC genes might have evolved with increasing cellular complexity.