F-ACTIN BUNDLING ACTIVITY OF TETRAHYMENA ELONGATION-FACTOR 1-ALPHA ISREGULATED BY CA2+ CALMODULIN

Translation elongation factor 1 alpha (EF-1 alpha) catalyzes the GTP-d ependent binding of aminoacyl-tRNA to the ribosome, Previously, Tetrah ymena 14-nm filament-associated protein was identified as EF-1 alpha [ Kurasawa et al, (1992) Exp. Cell Res. 203, 251-258]. This and several other studies suggest that EF-1 alpha functions not only in translatio n but also in regulation of some part of the cytoskeleton. Tetrahymena EF-1 alpha bound to F-actin and induced bundling of F-actin. We inves tigated the effects of GTP/GDP and Ca2+/calmodulin on F-actin bundling activity of EF-1 alpha. The presence of GTP, GDP, or guanylyl-imidodi phosphate (GMP-PNP) slightly decreased the amount of EF-1 alpha which bound to F-actin, but each had virtually no effect on the F-actin bund ling activity. The formation of F-actin bundles by EF-1 alpha was Ca2-insensitive. In the absence of Ca2+, calmodulin did not bind to EF-1 alpha and F-actin. On the other hand, in the presence of Ca2+, calmodu lin directly bound to EF-1 alpha but did not have any serious influenc e on EF-1 alpha/F-actin binding. Under the conditions, electron micros copy demonstrated that Ca2+/calmodulin completely inhibited the F-acti n bundling by EF-1 alpha. These results indicate that Ca2+/calmodulin regulates the F-actin bundling activity of EF-1 alpha without inhibiti on of the binding between EF-1 alpha and F-actin.