RAPID TURNOVER OF PHOSPHATIDYLINOSITOL 3-PHOSPHATE IN THE GREEN-ALGA CHLAMYDOMONAS-EUGAMETOS - SIGNS OF A PHOSPHATIDYLINOSITIDE 3-KINASE SIGNALING PATHWAY IN LOWER PLANTS

When Chlamydomonas eugametos gametes were incubated in carrier-free [P-32]P-1, the label was rapidly incorporated into PtdInsP and PtdInsP(2) and, after reaching a maximum within minutes, was chased out by recirculating unlabelled P-1 in the cell. This pulse-chase labelling pattern reflects their rapid turnover. In contrast, P-32 incorporation into the structural lipids was slow and continued for hours. Of the radioactivity in the PtdInsP spot, 15% was in PtdIns3P and the rest in PtdIns4P, and of that in the PtdInsP(2) spot, 1% was in PtdIns(3,4)P-2 and the rest in PtdIns(4,5)P-2, confirming the findings by Irvine, Letcher, Stephens and Musgrave [(1992) Biochem. J. 281, 269-266]. When cells were labelled with carrier-free [P-32]P-i, both PtdInsP isomers incorporated label in a pulse-chase-type pattern, demonstrating for the first time in a plant or animal system that D-3 polyphosphoinositides turn over rapidly in non-stimulated cells, with kinetics similar to those shown by the D-4 isomers. In animal systems such lipids are already established as signalling molecules, and the data suggest that a similar role must be sought for them in lower plants such as Chlamydomonas.