INTERACTION OF RHO1P TARGET BNI1P WITH F-ACTIN-BINDING ELONGATION-FACTOR 1-ALPHA - IMPLICATION IN RHO1P-REGULATED REORGANIZATION OF THE ACTIN CYTOSKELETON IN SACCHAROMYCES-CEREVISIAE

The RHO1 gene encodes a homolog of mammalian RhoA small G protein in t he yeast Saccharomyces cerevisiae. We have shown that Bni1p is one of the downstream targets of Rho1p and regulates reorganization of the ac tin cytoskeleton through the interaction with profilin, an actin monom er-binding protein. A Bni1p-binding protein was affinity purified from the yeast cytosol fraction and was identified to be Tef1p/Tef2p, tran slation elongation factor 1 alpha (EF1 alpha). EF1 alpha is an essenti al component of the protein synthetic machinery and also possesses the actin filament (F-actin)binding and -bundling activities. EF1 alpha b ound to the 186 amino acids region of Bni1p, located between the FH1 d omain, the proline-rich profilin-binding domain, and the FH2 domain, o f which function is not known. The binding of Bni1p to EF1 alpha inhib ited its F-actin-binding and -bundling activities. The BNI1 gene delet ed in the EF1 alpha-binding region did not suppress the bni1 bnr1 muta tion in which the actin organization was impaired. These results sugge st that the Rho1p-Bni1p system regulates reorganization of the actin c ytoskeleton through the interaction with both EF1 alpha and profilin.