Localization to the rhizoid tip implicates a Fucus distichus Rho family GTPase in a conserved cell polarity pathway

Generation and expression of cell polarity in brown algal zygotes of the Fucales involve regulation of the actin cytoskeleton and localized secretion. We used degenerate PCR to isolate cDNAs that encode two small GTPases, FdRac1 and FdRab8, from zygotes of Fucus distichus (L.) Powell. Sequence analysis placed FdRac1 in the Rho family, which regulates actin, and FdRab8 in the Rab family, which regulates vesicle transport. As expected, bacterially expressed forms of both proteins bound GTP in vitro. When expressed in budding yeast, FdRac1 showed some functional overlap with CDC42, the Saccharomyces cerevisiae Rho family gene required for yeast cell polarity. Immunolocalization revealed an asymmetric distribution of FdRac1 in polarized zygotes and embryos, with FdRac1 concentrated at or near the growing tip of the algal rhizoid. Our data support the hypothesis that FdRac1 regulates algal cell polarity, possibly via the actin cytoskeleton. Because brown algae belong to the heterokont group, which diverged from other groups early in eukaryotic evolution, we argue that the Rho family function of regulating cell polarity is ancient and may extend throughout the eukaryotes.