The phosphoinositide binding Phox Homology Domain of Class II Phosphoinositide 3-Kinases similar to the Drosophila PI3K_68D protein
The PX domain is a phosphoinositide (PI) binding module present in many proteins with diverse functions. The Phosphoinositide 3-Kinase (PI3K) family of enzymes catalyzes the phosphorylation of the 3-hydroxyl group of the inositol ring of phosphatidylinositol. PI3Ks play an important role in a variety of fundamental cellular processes, including cell motility, the Ras pathway, vesicle trafficking and secretion, immune cell activation and apoptosis. PI3Ks are divided into three main classes (I, II, and III) based on their substrate specificity, regulation, and domain structure. Class II PI3Ks preferentially use PI as a substrate to produce PI3P, but can also phosphorylate PI4P to produce PI(3,4)P2. They function as monomers and do not associate with any regulatory subunits. Class II enzymes contain an N-terminal Ras binding domain, a lipid binding C2 domain, a PI3K homology domain of unknown function, an ATP-binding cataytic domain, a PX domain, and a second C2 domain at the C-terminus. PI3K_68D is a novel PI3K which is widely expressed throughout the Drosophila life cycle. In vitro, it has been shown to phosphorylate PI and PI4P. It is involved in signaling pathways that affect pattern formation of Drosophila wings.
Comment:Two basic residues are key in binding with phosphoinositides: one forms hydrogen bonds with the 3-phosphate of PI(3)P and another forms hydrogen bonds with the 4-and 5-hydroxyl groups of PI(3)P.