WASP and N-WASP in human platelets differ in sensitivity to protease calpain

Mutations of Wiskoft-Aldrich syndrome protein (WASP) underlie the severe th rombocytopenia and immunodeficiency of the Wiskott-Aldrich syndrome. WASP, a specific blood cell protein, and its close homologue, the broadly distrib uted N-WASP, function in dynamic actin polymerization processes. Here it is demonstrated that N-WASP is expressed along with WASP, albeit at low level s, in human blood cells. The presence of approximately 160 nmol/L rapidly a cting N-WASP molecules may explain the normal capacity of WASP-negative pat ient platelets for early agonist-induced aggregation and filopodia formatio n. Ex vivo experiments revealed a significant difference between WASP and N -WASP in sensitivity to calpain, the Ca++-dependent protease activated in a gonist-stimulated platelets. Through the use of a series of calpain-contain ing broken cell systems, it is shown that WASP is cleaved in a Ca++-depende nt reaction inhibitable by calpeptin and E64d and that N-WASP is not cleave d, suggesting that the cleavage of WASP by calpain functions in normal plat elets as part of a Ca++-dependent switch mechanism that terminates the surf ace projection phase of blood cell activation processes. (C) 2001 by The Am erican Society of Hematology.