A new pathway for synthesis of phosphatidylinositol-4,5-bisphosphate

Abstract

Phosphatidylinositol-4,5-bisphosphate (PtdIns-4,5-P2), a key molecule in the phosphoinositide signalling pathway, was thought to be synthesized exclusively by phosphorylation of PtdIns-4-P at the D-5 position of the inositol ring. The enzymes that produce PtdIns-4,5-P2 in vitro fall into two related subfamilies (type I and type II PtdInsP-5-OH kinases, or PIP(5)Ks) based on their enzymatic properties and sequence similarities'. Here we have reinvestigated the substrate specificities of these enzymes. As expected, the type I enzyme phosphorylates PtdIns-4-P at the D-5 position of the inositol ring. Surprisingly, the type 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 kinase, or PIP(4)K. The earlier error in characterizing the activity of the 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 previously thought not to exist in vivo, we find evidence for the presence of this lipid in mammalian fibroblasts, establishing a new pathway for PtdIns-4,5-P2 synthesis.

Rapid Accumulation of Phosphatidylinositol 4,5-Bisphosphate and Inositol 1,4,5-Trisphosphate Correlates with Calcium Mobilization in Salt-Stressed Arabidopsis

“…It was additionally noted that the quantity of PtdIns(4,5)P 2 in non-stressed plants was very low in comparison with yeast, algae, and mammalian cells. It is interesting that a radiolabeled species elutes where glycerophosphoinositol 5-phosphate would be detected (data not shown), although phosphatidylinositol 5-phosphate has only been reported in animal cells (Rameh et al, 1997). Additional analyses must be performed to determine whether this peak is derived from PtdIns(5)P and if this phosphoinositide is being used as a substrate for phosphoinositide kinases to produce PtdIns(4,5)P 2 .…”

Rapid Turnover Rate of Phosphoinositides at the Front of Migrating MDCK Cells

“…Further on this line, this kinetic simulation model will predict the effect of drugs targeting phosphoinositide-metabolizing enzymes on cell migration. As a possible precursor of PI(4,5)P 2 , we focused on PI(4)P in the plasma membrane because the quantity of PI(5)P was much lower than the level of PI(4)P in eukaryotes (see above) (Rameh et al, 1997). Although the majority of PI(4)P and its precursor PtdIns localized to the Golgi apparatus, and although it is known that type II (wortmannin-insensitive) and type III (wortmannin-sensitive) PI 4-kinases (␣ and ␤) are primarily localized to endomembranes such as the Golgi apparatus and endoplasmic reticulum, a small amount of PI(4)P also locates to the plasma membrane, due to the activity of type-III␣ PI 4-kinase .…”

Phosphoinositides in Constitutive Membrane Traffic

“…The actual substrate in the bovine brain PtdIns(4)P that was used for the original purification of the enzymes was a previously unidentified lipid, PtdIns(5)P. Thus these enzymes should be called PIP4Ks. PIP4K␣ can phosphorylate PtdIns(3)P and PtdIns(5)P but does not make PtdIns(3,4,5)P 3 (283,409) and will not phosphorylate PtdIns (106). The PIP4Ks are also not stimulated by PA (164).…”

“…Based on sequence relationships, the PIP5Ks were grouped into two families called types I and II. Subsequently, it was realized that type II kinases phosphorylated the D-4 position and not D-5 and that both type I and II enzymes also phosphorylated the D-3 position (283,409). Thus the PIP5KII proteins are more accurately called PIP4Ks.…”