Type II PtdInsP kinases: location, regulation and function.
The regulation of the synthesis of PtdIns(4,5)P2 is emerging as being as complex as we might expect from the multi-functional nature of this lipid. In the present chapter we focus on one aspect of inositide metabolism, which is the functions of the Type II PIPkins (Type II PtdInsP kinases). These are primarily PtdIns5P 4-kinases, although in vitro they will also phosphorylate PtdIns3P to PtdIns(3,4)P2. Thus they have three, not necessarily exclusive, functions: to make PtdIns(4,5)P2 by a quantitatively minor route, to remove PtdIns5P and to make PtdIns(3,4)P2 by a route that does not involve a Class I PtdIns 3-kinase. None of these three possible functions has yet been unambiguously proven or ruled out. Of the three isoforms, alpha and beta are widely expressed, the IIalpha being predominantly cytosolic and the IIbeta primarily nuclear. PIPkin IIgamma has a much more restricted tissue expression pattern, and appears to be localized primarily to intracellular vesicles. Here we introduce in turn each of the three Type II PIPkins, and discuss what we know about their localization, their regulation and their function.