Acts on phosphatidylinositol (PtdIns) in the first committed step in the production of the second messenger inositol-1,4,5,-trisphosphate. a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol) + ATP = a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol 4-phosphate) + ADP + H(+) Activated by Triton X-100, insensitive to inhibition by adenosine and inhibited by wortmannin (By similarity). The PI4K complex acts as a regulator of phosphatidylinositol 4-phosphate (PtdIns(4)P) synthesis. Interaction with TMEM150A regulates PtdIns(4)P synthesis (By similarity). Component of a phosphatidylinositol 4-kinase (PI4K) complex, composed of PI4KA, EFR3 (EFR3A or EFR3B), TTC7 (TTC7A or TTC7B) and FAM126 (FAM126A or FAM126B). Interacts with TMEM150A; regulating recruitment to the plasma membrane. Interacts with TTC7A. Localization to the plasma membrane is mediated by the PI4K complex and association with EFR3 (EFR3A or EFR3B), TTC7 (TTC7A or TTC7B) and FAM126 (FAM126A or FAM126B). Localization to the plasma membrane is regulated by TMEM150A. Early embryonic lethality. Conditional knockout mice show a dramatic depletion of cellular phosphatidylinositol 4-phosphate (PtdIns(4)P). Belongs to the PI3/PI4-kinase family. Type III PI4K subfamily.