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 (PubMed:8662589). The PI4K complex acts as a regulator of phosphatidylinositol 4-phosphate (PtdIns(4)P) synthesis (By similarity). 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. Detected in the brain, kidney and lung. Less intensely expressed in the small intestine, uterus, adrenal gland, heart, skeletal muscle, thymus, spleen and testis. In brain of prenatal day 18 embryos, the expression is detected throughout the mantle zone of fore-, mid-, and hind brain. In the cerebrum, the expression is intense in the cortical plate and weak in the ventricular zone. At P49, expressed in the gray matter of the entire brain by hippocampal pyramidal cells, dentate granule cells, and cerebellar granule cells and to a lower extent by olfactory mitral and granule cells and the cerebral cortex. Weakly expressed in the diencephalon and brain stem and not detected in the cerebellar medulla. Expression is much higher in the fetal brain than the adult brain, especially in the brain stem. Belongs to the PI3/PI4-kinase family. Type III PI4K subfamily.