Inositide evolution - towards turtle domination?

Abstract

When viewing the changes in our understanding of inositides over the last 20 years, it is difficult to know whether to be more impressed by the proliferation in the number of inositides themselves (e.g. seven polyphosphoinositol lipids, more than 30 inositol phosphates), or by the number of functions for each. This review will focus on two specific aspects of this diversity: the evolution of the polyphosphoinositides, and the synthesis and functions of the higher inositol phosphates.

Figure 1. myo-Inositol

myo-Inositol is depicted as a Haworth projection (A) as a more accurate spatial representation (B), and schematically as a turtle (Agranoff, 1978) (C).

Figure 2. Eukaryotic inositol phosphate metabolism

Adapted from Irvine & Schell (2001). This is a simplified version, designed to illustrate only specific points discussed in the text. Thus, no phosphatase reactions (other than the conversion of Ins(1,3,4,5)P₄ to Ins(1,3,4)P₃) are shown, and no pathways from higher plants are illustrated. Thick black arrows indicate reactions universal (as far as we know) in eukaryotes, and thinner coloured arrows illustrate reactions confined to specific groups of organisms (again, as far as we know, and within the limits of the discussion in the text): purple, slime moulds; blue, animals; red, yeast. For fuller versions of these pathways, see Irvine & Schell, 2001; Shears, 2001). Note that the isomeric configuration of (PP)₂InsP₄ is not yet known for animal cells (Irvine & Schell, 2001).