The sequence of phosphatidylinositol-4-phosphate 5-kinase defines a novel family of lipid kinases.

Phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) occupies an essential position in the phosphoinositide signal transduction cascades as the precursor to second messengers and is thought to regulate many cellular proteins directly. The final step in the synthesis of PtdIns(4,5)P2 is the phosphorylation of PtdIns(4)P- by PtdIns(4)P 5-kinase (PIP5K). Using peptide sequences from a purified PIP5K, a cDNA for a human placental PIP5K was isolated and sequenced. Expression of this cDNA in Escherichia coli produced an active PIP5K. Surprisingly, the sequence of this PIP5K has no homology to known PtdIns kinases or protein kinases. However, the PIP5K is homologous to the Saccharomyces cerevisiae proteins Fab1p and Mss4p.