SH2-mediated steric occlusion of the C2 domain regulates autoinhibition of SHIP1 inositol 5-phosphatase.

The Src homology 2 (SH2) domain containing inositol polyphosphate 5-phosphatase 1 (SHIP1) is an immune cell specific enzyme that regulates phosphatidylinositol-(3,4,5)-trisphosphate signaling at the plasma membrane following receptor activation. SHIP1 plays an important role in processes such as directed cell migration, endocytosis, and cortical membrane oscillations. Alterations in SHIP1 expression have been shown to perturb myeloid cell chemotaxis and differentiation. In the brain, SHIP1 regulate microglial cell behaviors, which has been linked to Alzheimer's disease. Understanding the structural and functional relationships of SHIP1 is critical for developing ways to modulate SHIP1 membrane localization and lipid phosphatase activity during immune cell signaling. Recently, we discovered that the N-terminal SH2 domain of SHIP1 suppresses lipid phosphatase activity. SHIP1 autoinhibition can be relieved through interactions with receptor-derived phosphotyrosine (pY) peptides presented on membranes or in solution. Using hydrogen-deuterium exchange mass spectrometry (HDX-MS) we identified intramolecular contacts between the N-terminal SH2 domain and CBL1 motif of the C2 domain that limit SHIP1 membrane localization and activity. Single molecule measurements of purified SHIP1 on supported lipid bilayers and in neutrophil-like cells support a model in which the SH2 domain blocks membrane binding of the central catalytic module. Mutations that disrupt autoinhibition enhance the membrane binding frequency and increase the catalytic efficiency of SHIP1. Although dimerization of SHIP1 enhances membrane localization and the apparent phosphatase activity, it is not required for SHIP1 autoinhibition. Overall, our results provide new insight concerning SHIP1's structural organization, membrane binding dynamics, and the mechanism of autoinhibition.