Catalytic domain of Type III Phosphoinositide 4-kinase alpha
PI4Ks catalyze the transfer of the gamma-phosphoryl group from ATP to the 4-hydroxyl of the inositol ring of D-myo-phosphatidylinositol (PtdIns) to generate PtdIns(4)P, the major precursor in the synthesis of other phosphoinositides including PtdIns(4,5)P2, PtdIns(3,4)P2, and PtdIns(3,4,5)P3. Two isoforms of type III PI4K, alpha and beta, exist in most eukaryotes. PI4KIIIalpha is a 220 kDa protein found in the plasma membrane and the endoplasmic reticulum (ER). The role of PI4KIIIalpha in the ER remains unclear. In the plasma membrane, it provides PtdIns(4)P, which is then converted by PI5Ks to PtdIns(4,5)P2, an important signaling molecule. Vertebrate PI4KIIIalpha is also part of a signaling complex associated with P2X7 ion channels. The yeast homolog, Stt4p, is also important in regulating the conversion of phosphatidylserine to phosphatidylethanolamine at the ER and Golgi interface. Mammalian PI4KIIIalpha is highly expressed in the nervous system. The PI4K catalytic domain family is part of a larger superfamily that includes the catalytic domains of other kinases such as the typical serine/threonine/tyrosine protein kinases (PKs), aminoglycoside phosphotransferase, choline kinase, and RIO kinases.
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