A NEW PATHWAY FOR SYNTHESIS OF PHOSPHATIDYLINOSITOL-4,5-BISPHOSPHATE

Phosphatidylinositol-4,5-bisphosphate (PtdIns-4,5-P-2), a key molecule in the phosphoinositide signalling-pathway, was thought to be synthes ized exclusively by phosphorylation of PtdIns-4-P at the D-5 position of the inositol ring. The enzymes that produce PtdIns-4,5-P-2 in vitro fall into two related subfamilies (type I and type II PtdInsP-5-OH ki nases, or PIP(5)Ks) based on their enzymatic properties and sequence s imilarities(1). Here we have reinvestigated the substrate specificitie s of these enzymes. As expected, the type I enzyme phosphorylates PtdI ns-4-P at the D- 5 position of the inositol ring. Surprisingly, the ty pe II enzyme, which is abundant in some tissues, phosphorylates PtdIns -5-P at the D-4 position, and thus should be considered as a 4-OH kina se, or PIP(4)K. The earlier error in characterizing the activity of th e type II enzyme is due to the presence of contaminating PtdIns-5-P in commercial preparations of PtdIns-4-P. Although PtdIns-5-P was previo usly thought not to exist in vivo, we find evidence for the presence o f this lipid in mammalian fibroblasts, establishing a new pathway for PtdIns-4,5-P-2 synthesis.