WIP Is a Chaperone for Wiskott-Aldrich Syndrome Protein.

WASP, the product of the gene mutated in Wiskott-Aldrich syndrome (WAS), plays a critical role in T cell activation and actin reorganization. WIP is a WASP-interacting protein that binds to Ena-VASP homology 1 (EVH1) domain of WASP. We previously reported functional roles of WIP in the recruitment and activation of WASP in T cells. To investigate a role of WIP in WASP degradation and understand the reason why most missense mutations in WAS patients are in EVH1 domain of WASP, we examined WASP levels in WIP knockout (WIP−/−) T cells and mechanisms of WASP degradation following T cell receptor (TCR) ligation. WASP protein levels, but not mRNA levels, were severely diminished in T cells from WIP−/− mice and were restored by introduction of full-length or WASP binding domain of WIP. A fraction of WASP was cleaved by Ca-dependent protease calpain, ubiquitinated and degraded by the proteasome following TCR ligation. Inducible expression of WASP binding domain of WIP increased WASP levels in vivo and recombinant WIP protein protected WASP from degradation by calpain in vitro. Treatment with the proteasome inhibitors MG132 and bortezomib increased WASP protein levels in T cells from WIP−/− mice and in T and B lymphocytes from two WAS patients with missense mutations (R86H and T45M) that disrupted WIP binding. The calpain inhibitor calpeptin also increased WASP protein levels in activated T cells and B cells from the WAS patients. Despite its ability to increase WASP levels, proteasome inhibitors failed to correct the impaired IL-2 gene expression and low F-actin content in T cells from the R86H WAS patient (published in