Coronin promotes the rapid assembly and cross-linking of actin filaments and may link the actin and microtubule cytoskeletons in yeast

Coronin is a highly conserved actin-associated protein that until now has h ad unknown biochemical activities. Using microtubule affinity chromatograph y, we coisolated actin and a homologue of coronin, Crn1p, from Saccharomyce s cerevisiae cell extracts. Crn1p is an abundant component of the cortical actin cytoskeleton and binds to F-actin with high affinity (K-d 6 x 10(-9) M). Crn1p promotes the rapid barbed-end assembly of actin filaments and cro ss-links filaments into bundles and more complex networks, but does not sta bilize them. Genetic analyses with a crn1 Delta deletion mutation also are consistent with Crn1p regulating filament assembly rather than stability. F ilament crosslinking depends on the coiled coil domain of Crn1p, suggesting a requirement for Crn1p dimerization. Assembly-promoting activity is indep endent of cross-linking and could be due to nucleation and/or accelerated p olymerization. Crn1p also binds to microtubules in vitro, and microtubule b inding is enhanced by the presence of actin filaments. Microtubule binding is mediated by a region of Crn1p that contains sequences (not found in othe r coronins) homologous to the microtubule binding region of MAP1B. These ac tivities, considered with microtubule defects observed in crn1 Delta cells and in cells overexpressing Crn1p, suggest that Crn1p may provide a functio nal link between the actin and microtubule cytoskeletons in yeast.