Regulation and dysregulation at a critical hematopoietic junction: The human WASP-WIP complex

The complex formed between Wiskott-Aldrich syndrome protein (WASP) and WASP-Interacting Protein (WIP) is a potent regulator of cytoskeletal changes in hematopoietic cells. Mutations in the WASP N-terminal domain cause the primary immunodeficiencies Wiskott-Aldrich syndrome (WAS) and X-linked thrombocytopenia (XLT). Using NMR we determine the structure of the WASP/WIP complex and provide a first molecular view of this key biochemical junction. The central feature of this complex is the extensive binding interface formed by four WIP epitopes that wrap around the canonical EVH1 domain. Phosphoregulation of the WIP chaperone function occurs on two tyrosine residues, and not a distal serine residue as suggested earlier, and involves selective dissociation of the fourth epitope (epi^IV), thereby exposing two established WASP ubiquitylation sites. Single-residue WAS-inducing mutations with mild phenotypes all influence the same WASP-epi^IV interface, suggesting this is the molecular mechanism behind WAS. This structural viewpoint of WASP/WIP biology creates a much-needed molecular context for understanding hematopoietic cytoskeletal regulation in homeostasis and in WAS/XLT and is expected to be invaluable in the search for new therapeutic approaches to these rare diseases.