Testicular protein kinase 1 (TESK1) is a serine/threonine kinase that phosphorylates cofilin and induces actin cytoskeletal reorganization. The kinase activity of TESK1 is stimulated by integrin-mediated signaling pathways, but the mechanism of regulation has remained unknown. By using the yeast two-hybrid system, we identified 14-3-3β to be the binding protein of TESK1. Specific interaction between TESK1 and 14-3-3β became evident in in vitro and in vivo co-precipitation assays. 14-3.3β interacts with TESK1 through the C-terminal region of TESK1 and in a manner dependent on the phosphorylation of Ser-439 within an RXXSXP motif. Binding of 14-3-3β inhibited the kinase activity of TESK1. During cell spreading on fibronectin, the TESK1/14-3-3β interaction significantly decreased, in a time course that inversely correlated with increase in TESK1 kinase activity. Thus, the dissociation of 14-3-3β from a TESK1/14-3-3β complex is likely to be involved in the integrin-mediated TESK1 activation. In HeLa cells, TESK1, together with 14-3-3β, accumulated at the cell periphery when cells were plated on fibronectin, whereas they were diffusely distributed in the cytoplasm in the case of non-stimulated cells. We propose that 14-3-3β plays important roles in regulating the kinase activity of TESK1 and localizing TESK1 to cell adhesion sites following integrin stimulation.
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