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bioRxiv · 10.1101/096008

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

Christensen, J. R.·Glista, M. J.·Mueller, D. M.·Li, Y.·Sees, J. A.·Skau, C. T.·Mets, L. J.·Avasthi, P.·Kovar, D. R.
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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.

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Christensen, J. R., Glista, M. J., Mueller, D. M., Li, Y., Sees, J. A., Skau, C. T., Mets, L. J., Avasthi, P., Kovar, D. R.. 2016-12-21. Chlamydomonas reinhardtii formin and profilin are optimized for acute rapid actin filament assembly. https://doi.org/10.1101/096008

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