The pleckstrin homology domain of the Wiskott-Aldrich syndrome protein is involved in the organization of actin cytoskeleton

In this study, we investigated the role of the pleckstrin homology (PH) dom ain of the Wiskott-Aldrich syndrome protein (WASP) in the regulation of act in cytoskeleton, which is defective in patients with Wiskott-Aldrich syndro me (WAS) and X-linked thrombocytopenia (XLT). Overexpression of the WASP in COS-7 cells cultured in the presence of fetal calf serum (FCS) resulted in large cluster formation of polymerized actin and WASP in the cytoplasm. In contrast, when the WASP transfected cells were cultured in the absence of FCS, activation with PMA or EGF was required to induce cluster formation. O verexpression of WASP with a missense mutation in the N-terminus of the PH domain failed to induce the large cluster formation in COS-7 cells even in the presence of FCS. We also found that phosphatidylinositol 4,5-bisphospha te (PIP,), which is known to regulate the actin cytoskeleton, binds to the PH domain of WASP, and the binding was abolished by the introduction of a m issense mutation into the N-terminus but not the C-terminus of the PH domai n. Together with the observations that most of the missense mutations obser ved in patients with WAS and XLT are located within the PH domain, these re sults indicate that the PH domain of WASP plays important roles in the regu lation of actin cytoskeleton and suggested that the binding of PIP, to the PH domain is necessary for WASP to function properly. (C) 1999 Academic Pre ss.