Alternative metabolic fates of phosphatidylinositol produced by PI-synthase isoforms in Arabidopsis thaliana
Citable Link (URL):http://resolver.sub.uni-goettingen.de/purl?gs-1/5700
Phosphatidylinositol (PtdIns) is an important precursor for inositol-containing lipids, including polyphosphoinositides, which have multiple essential functions in eukaryotic cells. It was previously proposed that different regulatory functions of inositol-containing lipids may be performed by independent lipid-pools, however, it remains unclear how such subcellular pools are established and maintained. Here, a previously-uncharacterised Arabidopsis gene-product with similarity to the known Arabidopsis PI-synthase, PIS1, is shown to be an active enzyme, PIS2, capable of producing PtdIns in vitro. PIS1 and PIS2 diverged slightly in substrate preferences for cytidinediphospho-diacylglycerol species differing in fatty acid composition, PIS2 preferring unsaturated substrates in vitro. Transient expression of fluorescence-tagged PIS1 or PIS2 in onion epidermal cells indicates localisation of both enzymes in the endoplasmic reticulum and, possibly, Golgi, as was reported previously for fungal and mammalian homologues. Constitutive ectopic overexpression of PIS1 or PIS2 in Arabidopsis plants resulted in elevated levels of PtdIns in leaves. PIS2-overexpressors additionally exhibited significantly-elevated levels of PtdIns-4-phosphate and PtdIns-4,5-bisphosphate, whereas polyphosphoinositides were not elevated in plants overexpressing PIS1. In contrast, PIS1-overexpressors contained significantly-elevated levels of diacylglycerol and phosphatidylethanolamine, an effect not observed in plants overexpressing PIS2. Biochemical analysis of transgenic plants regarding fatty acids associated with relevant lipids indicates that lipids increasing with PIS1-overexpression were enriched in saturated or monounsaturated fatty acids, whereas lipids increasing with PIS2-overexpression, including polyphosphoinositides, contained more unsaturated fatty acids. The data indicate that PtdIns-populations originating from different PI-synthase isoforms may enter alternative routes of metabolic conversion, possibly based on specificity and immediate metabolic context of the biosynthetic enzymes.