FRL, a Novel Formin-Related Protein, Binds to Rac and Regulates Cell Motility and Survival of Macrophages

Abstract

We have isolated a cDNA, frl(formin-related gene in leukocytes), a novel mammalian member of the formin gene family. The frlcDNA encodes a 160-kDa protein, FRL, that possesses FH1, FH2, and FH3 domains that are well conserved among other Formin-related proteins. An FRL protein is mainly localized in the cytosol and is highly expressed in spleen, lymph node, and bone marrow cells. Formin-related genes and proteins have been reported to play crucial roles in morphogenesis, cell polarity, and cytokinesis through interaction with Rho family small GTPases. FRL binds to Rac at its N-terminal region including the FH3 domain and associates with profilin at the FH1 domain. In a macrophage cell line, P388D1, overexpression of a truncated form of FRL containing only the FH3 domain (FH3-FRL) strongly inhibited cell adhesion to fibronectin and migration upon stimulation with a chemokine. Moreover, expression of the truncated FH3-FRL protein resulted in apoptotic cell death of P388D1 cells, suggesting that the truncated FH3-FRL protein may interfere with signals of FRL. Overexpression in the P388D1 cells of full-length FRL or of the truncated protein containing the FH3 and FH1 domains, with simultaneous expression of the truncated FH3-FRL protein, blocked apoptotic cell death and inhibition of cell adhesion and migration. These results suggest that FRL may play a role in the control of reorganization of the actin cytoskeleton in association with Rac and also in the regulation of the signal for cell survival.

ACKNOWLEDGMENTS

We thank T. Takenawa (University of Tokyo, Tokyo, Japan) for the gift of pGEX-profilin I and pGEX-profilin II, and N. Watanabe (University of Kyoto, Kyoto, Japan) for pGEX-RhoA, pGEX-Rac1, and pGEX-Cdc42Hs vectors. We are also grateful to Peter D. Burrows for critical reading of the manuscript.