Structural and biochemical insight into the inositol polyphosphate 4-phosphatase family of proteins

Abstract

<p>Inositol polyphosphate 4-phosphatases (IP4Ps) are lipid phosphatases that remove the 4-position phosphate from the lipid second messenger phosphatidylinositol 3,4-bis-phosphate converting it to phosphatidylinositol 3-phosphate. There are 2 families of IP4P's that are 37% homologous to each other denoted type I and type II. Previously, several different isoforms of both type I and type II IP4P had been identified. These include the beta-spliceoforms of both type I and II that contained a hydrophobic tail instead of a hydrophilic tail and a novel deletion spliceoform found in brain tissue that is approximately 1kDa smaller than type Ialpha called type Ialpha2. In this dissertation I first have identified a novel splice variant of type I IP4P. IP4P type Ialpha3 is an 110kDa splice variant of type I IP4P's that contains an additional 40 amino acids creating a PEST region. The presence of the additional PEST region could play a role in proteolytic regulation of type I IP4P. This novel spliceoform is predominantly located in T-cells and in other hematopoietic cells. IP4P Ialpha3 could have specialized roles in immune related functions. IP4P are lipid phosphatases, therefore they must be able to get to membranes to have access to their substrate. Subsequently the characterization of a lipid-binding C2 domain located on the N-terminal end of both type I and type II IP4Ps is performed. These C2 domains also bind calcium but in the presence of calcium lipid binding is decreased. In addition, we identify a short spliceoform of type II IP4P which we denote Stubby. This spliceoform lacks the phosphatase domain and consists primarily of a C2 domain. The C2 domain of Stubby is not complete, by sequence homology revealed different substrate specificity. In a lipid overlay, the IP4P type I C2 domain and Stubby prefer to bind phosphatidylinositol 3,5-bisphosphate whereas the type II C2 domain prefers phosphatidylinositol 3,4,5-tris-phosphate. This suggests that different forms of IP4Ps are localized to different places in the cell.</p>