ABSTRACT
Chlamydomonas reinhardtii is a unicellular green alga that appears less dependent upon a conventional actin cytoskeleton than other eukaryotes, in part due to overlapping functions of a second non-conventional actin. One network that contains exclusively conventional F-actin is the fertilization tubule, a mating structure at the apical cell surface in gametes. Therefore, Chlamydomonas is an excellent system to investigate how actin polymerization is regulated in space and time. Chlamydomonas expresses a profilin (CrPRF), and a formin (CrFor1) that we have characterized for the first time. We found that unlike typical profilins, CrPRF prevents unwanted actin assembly by strongly inhibiting both F-actin nucleation and barbed end elongation at equimolar concentrations to actin. However, CrFor1 is able to stimulate rapid actin filament assembly of CrPRF-bound actin. CrPRF further favors CrFor1-mediated actin assembly by potently inhibiting Arp2/3 complex-mediated actin assembly. The small molecule formin inhibitor SMIFH2 prevents fertilization tubule formation in gametes, suggesting that mating is a primary function of CrFor1. Together, these findings indicate that CrFor1 and CrPRF cooperate to selectively and rapidly assemble F-actin at the right time and place.
