The role of phosphatidylinositol transfer proteins in phosphoinositide signalling

The classical model of the phosphoinositide (PI) signalling pathway postulates that the substrate lipid, phosphatidylinositol (PtdIns), is sequentially phosphorylated by PI kinases to form PtdIns 4,5-bisphosphate (PtdInsP2), before being cleaved by phospholipase C (PLC) to generate two second messengers, inositol 1,4,5-trisphosphate and diacylglycerol (DAG). PtdIns is thought to be embedded in the membrane lipid bilayer throughout this process. However, several reports have strongly suggested that a PtdIns transfer protein (PtdInsTP) is not only essential but may also be a co-factor in this pathway. The experiments described in this thesis address a key aspect of this suggestion. The specific hypothesis to be tested is that the mammalian PtdInsTPs PITPα and PITPβ bind one or more Ptdlns kinases in order to present bound Ptdlns for phosphorylation. The results presented in this thesis provide evidence that Ptdlns bound to PITP is a direct substrate for PI kinases. Recombinant PITP was found to co-purify with PI kinases, which recognised and phosphorylated PITP-bound Ptdlns to produce Ptdlns 4-phosphate and PtdInsP2-. Studies of a mutant PITP provided additional evidence for the above conclusion and also revealed that the co-purification of PITP and PI kinases was not dependent on the ability of PITP to bind Ptdlns. Liquid chromatographic analyses showed that PITP and PI kinases behaved as a complex. Furthermore, PLC and DAG kinase activities were also detected in the PITP-PI kinase complex. These studies provide strong support for the hypothesis that PITP is not only a component of a multi-enzyme signalling complex but that it also presents bound Ptdlns sequentially to the enzymes in a signalling cascade. Comparative studies were carried out on the two (alpha and beta) isoforms of PITP. Despite their different subcellular distribution, both isoforms were able to (i) transfer Ptdlns in vitro, (ii) reconstitute PLC signalling in permeabilised cells, and (iii) bind and present substrate to PI kinases in vitro. The biological significance of these results is discussed in the light of apparently complementary and conflicting studies by other groups