Properties of Acyl-(Acyl-Carrier Protein): Glycerol-3-Phosphate Acyltransferase from Greening Squash Cotyledons

Abstract

Fatty acid and molecular composition analyses of the lipids from higher plants indicate that a proportion of saturated molecular species of phosphatidylglycerol (PG) from plastids are well correlated with the chilling sensitivity of plants1–3. The thermotrophic phase behavior of aqueous dispersions of leaf lipids suggests that only PG from chilling-sensitive plants, but no other lipid from either chilling-sensitive or chillingresistant plants, undergoes phase transition at room temperature or above. The major saturated molecular species of PG are sn-1, 2-dipalmitoyl and 1-palmitoyl-2-(trans-3)hexadecenoyl. Since in plastid PG either palmitic acid (16: 0) or trans-3-hexadecenoic acid (16: lt) are esterified to the C-2 position of glycerol backbone, the proportion of 16: 0 at the C-1 position is equal to that of the saturated molecular species. Acyl-(acyl-carrier protein): glycerol-3-phosphate acyltransferase (hereinafter referred to as acyltransferase) in plastids carries out the first step of lipid biosynthesis in the chloroplasts i.e. esterification at the C-1 position of glycerol-3-phosphate5,6. We speculated that the substrate selectivity of this enzyme directs the fatty acid composition at the C-1 position and therefore determines the molecular species composition. To test this hypothesis, we purified the enzyme from greening cotyledons of squash, a chilling-sensitive plant, and examined its substrate selectivity.