14-3-3ζ Mediates Integrin-induced Activation of Cdc42 and Rac

Integrin-induced cytoskeletal reorganizations are initiated by Cdc42 and Rac1 but little is known about mechanisms by which integrins activate these Rho GTPases. 14-3-3 proteins are adaptors implicated in binding and regulating the function and subcellular location of numerous signaling molecules. In platelets, the 14-3-3 isoform interacts with the glycoprotein (GP) Ib subunit of the adhesion receptor GP Ib-IX. In this study, we show that integrin-induced activation of Cdc42, activation of Rac, cytoskeletal reorganizations, and cell spreading were inhibited in Chinese hamster ovary cells expressing full-length GP Ib compared with GP Ib lacking the 14-3-3 binding site. Activation of Rho GTPases and cytoskeletal reorganizations were restored by expression of 14-3-3. Spreading in cells expressing truncated GP Ib was inhibited by co-expressing a chimeric receptor containing interleukin 2 receptor and GP Ib cytoplasmic domain. These results identify a previously unrecognized function of 14-3-3, that of mediating integrin-induced signaling. They show that 14-3-3 mediates Cdc42 and Rac activation. They also reveal a novel function of platelet GP Ib-IX, that of regulating integrininduced cytoskeletal reorganizations by sequestering 14-3-3. Signaling across integrins initiates changes in cell behavior such as spreading, migration, differentiation, apoptosis, or cell division. Thus, introduction of the 14-3-3 binding domain of GP Ib into target cells might provide a method for regulating integrin-induced pathways in a variety of pathological conditions. The 14-3-3 family of proteins are expressed in all eukaryotic cells. There are at least seven highly conserved isoforms encoded by different gene products. These proteins have molecular weights of 29,000 -32,000 and bind numerous cytoplasmic and nuclear signaling molecules including signaling molecules such as Raf, protein kinase C, p130 Cas , BAD, and phosphatidylinositol 3-kinase (1-5), proteins such as Cdc25 and Wee1 that are involved in cell cycle control (6, 7), and proteins such as FKHRLl and DAF-16 (8, 9) that are involved in regulation of transcription.