Chlamydomonas reinhardtii formin FOR1 and profilin PRF1 are optimized for acute rapid actin filament assembly

Abstract

The regulated assembly of multiple filamentous actin (F-actin) networks from an actin monomer pool is important for a variety of cellular processes. Chlamydomonas reinhardtii is a unicellular green alga expressing a conventional and divergent actin that is an emerging system for investigating the complex regulation of actin polymerization. One actin network that contains exclusively conventional F-actin in Chlamydomonas is the fertilization tubule, a mating structure at the apical cell surface in gametes. In addition to two actin genes, Chlamydomonas expresses a profilin (PRF1) and four formin genes (FOR1-4), one of which (FOR1) we have characterized for the first time. We found that unlike typical profilins, PRF1 prevents unwanted actin assembly by strongly inhibiting both F-actin nucleation and barbed end elongation at equimolar concentrations to actin. However, FOR1 stimulates the assembly of rapidly elongating actin filaments from PRF1-bound actin. PRF1 further favors FOR1-mediated actin assembly by potently inhibiting Arp2/3 complex-mediated actin assembly. Furthermore, for1 and prf1-1 mutants, as well as the small molecule formin inhibitor SMIFH2, prevent fertilization tubule formation in gametes, suggesting that polymerization of F-actin for fertilization tubule formation is a primary function of FOR1. Together, these findings indicate that FOR1 and PRF1 cooperate to selectively and rapidly assemble F-actin at the right time and place.\n\nSUMMARY STATEMENTThe Chlamydomonas reinhardtii formin FOR1 initiates rapid assembly of fertilization tubule actin filaments from monomers associated with the actin-assembly inhibitor profilin PRF1.