RT Journal Article SR Electronic T1 Chlamydomonas reinhardtii formin and profilin are optimized for acute rapid actin filament assembly JF bioRxiv FD Cold Spring Harbor Laboratory SP 096008 DO 10.1101/096008 A1 Jenna R. Christensen A1 Michael J. Glista A1 David M. Mueller A1 Yujie Li A1 Jennifer A. Sees A1 Colleen T. Skau A1 Laurens J. Mets A1 Prachee Avasthi A1 David R. Kovar YR 2017 UL http://biorxiv.org/content/early/2017/02/09/096008.abstract AB 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.