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 2016 UL http://biorxiv.org/content/early/2016/12/21/096008.abstract AB Chlamydomonas reinhardtii is a unicellular green alga that appears less dependent upon its actin cytoskeleton than other cell types for fundamental processes like division and polarization. Null mutants of actin in Chlamydomonas, as well as treatment with actin depolymerizing drugs, have only mild defects. The reduced dependence on the actin cytoskeleton is now known to be due to the expression of a non-conventional actin in these cells which has both overlapping and unique functions. One structure that contains almost exclusively conventional F-actin is the mating fertilization tubule of gametes as actin depolymerizing drugs and conventional actin mutants have impaired fertilization tubule formation. Therefore, Chlamydomonas provides an excellent system to investigate regulation of the actin cytoskeleton, and in particular, how actin polymerization occurs at a very specific place and time. Interestingly, while it may contain a functionally minimal actin cytoskeleton, Chlamydomonas expresses both a profilin (CrPRF) and a formin (CrFor1). We have found that unlike typical profilins, CrPRF inhibits both F-actin nucleation and barbed end elongation at equimolar concentrations to actin. However, CrFor1 overcomes the low polymerizability of CrPRFbound actin and stimulates rapid actin filament assembly. CrPRF further favors CrFor1mediated actin assembly by potently inhibiting Arp2/3 complex-mediated actin assembly. Together, these findings suggest that CrFor1 and CrPRF work together to selectively and rapidly assemble F-actin at the right time and place. In vivo results demonstrate that this rapid actin assembly by CrFor1 is important specifically for fertilization tubule formation during mating, as formin inhibitor SMIFH2 inhibits formation of fertilization tubules in Chlamydomonas gametes.PRMproline rich motifTIRFtotal internal reflection fluorescence