The inositol polyphosphate 4-phosphatase (INPP4) family includes INPP4A, INPP4B and the two phosphatidylinositol (3,4,5)-trisphosphate (PI(3,4,5)P3)-dependent Rac exchangers (P-Rex) proteins, P-Rex1 and P-Rex2. All members of this family contain and INPP4-like domain and a conserved CX5R dual specificity phosphatase catalytic motif. However, catalytic activity has only been observed for INPP4A and INPP4B to date. INPP4A and INPP4B have overlapping substrate specificity with both preferentially hydrolysing the 4′ phosphate of PI(3,4)P2 generating PI(3)P. Additionally, INPP4A and INPP4B have can hydrolyse PI(4,5)P2 to PI(5)P, which antagonises receptor tyrosine kinase activity by removing the substrate required for phosphoinositide 3-kinase activity. The specificity of INPP4 activity is believed to be determined by the differing substrate specificity of their C2 lipid-binding domains, controlling recruitment to various cellular membranes.
The INPP4-like phosphatases are the final family of human phosphoinositide phosphatases without a solved active structural representative. Consequently, the mechanisms by which these proteins discriminate between different phosphoinositides and selectively cleave 4′ phosphates are largely unknown. Herein I will present the first reported structure of INPP4B and use mutagenesis and kinetic assays to characterise how INPP4B discriminates between phosphoinositides and the unique structural basis of 4′ phosphatase activity.