Characterization of phosphatidylinositol phosphate kinases from the moss Physcomitrella patens: PpPIPK1 and PpPIPK2

Phosphoinositides (PIs) play a major role in eukaryotic cells, despite being a minor component of most membranes. This is the first report on phosphoinositide metabolism in a bryophyte, the moss Physcomitrella patens. Moss PI composition is similar to other land plants growing under normal conditions. In contrast to the large number of PIPK genes present in flowering plants, the P. patens genome encodes only two type I/II PIPK genes: PpPIPK1 and PpPIPK2 which are very similar at both nucleotide and protein product levels. However, the expression of the two genes is differentially regulated and in vitro biochemical characterization shows that the resulting enzymes have different substrate specificities. PpPIPK1 uses PtdIns4P and PtdIns3P with similar preference and also metabolise PtdIns(3,4)P(2) to produce PtdIns(3,4,5)P(3), a PI not yet detected in intact plant cells. PpPIPK2 prefers PtdIns as substrate and is much less active towards PtdIns4P and PtdIns3P. Thus, PpPIPK2 shows properties reminiscent of both PtdInsP- and PtdIns-kinases. Moreover, a substitution of glutamic acid to alanine in the activation loop drastically reduced PpPIPK1 activity and altered the substrate specificity to PtdIns5P being the preferred substrate compared to PtdIns4P and PtdIns3P. These findings demonstrate that substrate specificity of plant PIPKs is determined in a plant-specific manner, which provides new insights into the regulatory modes of the PIPK activity in plants