Leucine zipper-mediated homodimerization of the p21-activated kinase-interacting factor, βPix -: Implication for a role in cytoskeletal reorganization

Pix, a p21-activated kinase-interacting exchange factor, is known to be involved in the regulation of Cdc42/ Pac GTPases, The 85-kDa beta Pix-a protein contains an Src homology 3 domain, the tandem Dbl homology and Pleckstrin homology domains, a proline-rich region, and a GIT1-binding domain. In addition to those domains, beta Pix-a also contains a putative leucine zipper domain at the C-terminal end. In this study, we demonstrate that the previously identified putative leucine zipper domain mediates the formation of beta Pix-a homodimers. Using in vittro and in vivo methodologies, we show that deletion of the leucine zipper domain is sufficient to abolish beta Pix-a homodimerization. In NIH3T3 fibroblast cells, expression of wild type beta Pix-a induces the formation of membrane ruffles. However, cells expressing the leucine zipper domain deletion mutant could not form membrane ruffle structures. Moreover, platelet derived growth factor-mediated cytoskeletal changes were completely blocked by the leucine zipper domain deletion mutant. The results suggest that the leucine zipper domain enables beta Pix-a to homodimerize, and homodimerization is essential for beta Pix-a signaling functions leading to the cytoskeletal reorganization.