Rho-family GTPases, such as Rho, Rac and Cdc42, are essential cytoskeletal regulators in higher eukaryotes, controlling reorganization of the actin network and polarized cell growth. Rho and Cdc42 are also present in S. cerevisiae and S. pombe. By contrast, Rac, which is implicated in control of lamellipodium formation and cell morphology, has no orthologue in these organisms. Why is this? Alex Andrianopoulos and co-workers provide a clue, identifying a Rac orthologue (CflB) in Penicillium marneffei— a fungus that switches between uninucleate yeast and multinucleate hyphal forms (see ). They show that CflB colocalizes with actin at septation sites and the tips of vegetative hyphal cells. Furthermore, by generating strains that express activated or dominant negative CflB (or lack it entirely), they demonstrate that the GTPase is necessary for cell polarization during hyphal growth but not yeast growth — unlike the Cdc42 orthologue, CflA. The authors also observe that CflB (but not CflA) is required for morphogenesis of conidiophores — complex multicellular structures that appear during asexual development of P. marneffei. The Rac GTPase thus appears to complement Cdc42 by providing aspects of polarity control associated with increasing cellular complexity.
