14-3-3ζ mediates integrin-induced activation of Cdc42 and Rac -: Platelet glycoprotein IB-IX regulates integrin-induced signaling by sequestering 14-3-3ζ

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 14-3-3zeta isoform interacts with the glycoprotein (GP) Ibalpha 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 Ibalpha compared with GP Ibalpha lacking the 14-3-3zeta binding site. Activation of Rho GTPases and cytoskeletal reorganizations were restored by expression of 14-3-3zeta. Spreading in cells expressing truncated GP Ibalpha was inhibited by co-expressing a chimeric receptor containing interleukin 2 receptor alpha and GP Ibalpha cytoplasmic domain. These results identify a previously unrecognized function of 14-3-3zeta, that of mediating integrin-induced signaling. They show that 14-3-3zeta mediates Cdc42 and Rac activation. They also reveal a novel function of platelet GP Ib-IX, that of regulating integrin-induced cytoskeletal reorganizations by sequestering 14-3-3zeta. Signaling across integrins initiates changes in cell behavior such as spreading, migration, differentiation, apoptosis, or cell division. Thus, introduction of the 14-3-3zeta binding domain of GP Ibalpha into target cells might provide a method for regulating integrin-induced pathways in a variety of pathological conditions.