Properties of the Plastidial Acyl-(Acyl-Carrier-Protein): Glycerol-3-Phosphate Acyltransferase from the Chilling-Sensitive Plant Squash (Cucurbita moschata)

To determine whether the plastidial acyl-(acyl-carrier-protein (ACP)): glycerol-3-phosphate acyltransferase from chilling-sensitive plants exhibits fatty acid selectivities different from those of resistant plants, we characterized this enzymic activity from the chilling-sensitive plant Cucurbita moschata. In squash cotyledons, the glycerol-3-phosphate acyltransferase (AT) occurred as three isomeric forms: one with an isoelectric point at pH 6.6 (ATI) and two at about pH 5.5 (AT2 and AT3). These isomers had approximately equal total activities in plastids. All three forms specifically directed acyl groups to the C-l position of glycerol-3-phosphate. However, ATI differed from the two other isomeric forms on the basis of kinetic data determined with different acyl-ACPs as substrates. These kinetic differences were reflected in the different fatty acid selectivities of the acyltransferases. ATI preferably utilized oleoyl groups in comparison to palmitoyl and stearoyl groups while AT2 and AT3 hardly discriminated between the acyl-ACP thioesters. However, the observed selectivity of ATI was significantly reduced by increasing the pH of the reaction mixture from 7.4 to 8.0, which is the stroma pH of illuminated chloroplasts. Consequently, the glycerol-3-phosphate acyltransferases from squash cotyledons could account for the high proportion of saturated acyl groups found at the C-l position of the plastidial phosphatidylglycerol from this plant.