Acts on phosphatidylinositol (PtdIns) in the first committed step in the production of the second messenger inositol-1,4,5,- trisphosphate. {ECO:0000269|PubMed:10101268, ECO:0000269|PubMed:23229899}. This is the function of PI4KA (phosphatidylinositol 4-kinase alpha, Ensembl gene identifier ENSG00000241973).