Characterization of reconstituted partially purified glycerophosphate acyltansferase from Escherichia coli

Structural characterization of ordered arrays of sn-glycerol-3-phosphate acyltransferase from Escherichia coli

Overproduction of the sn-glycerol-3-phosphate acyltransferase in Escherichia coli leads to incorporation of this integral membrane protein into ordered tubular arrays within the cell. Freeze-fracture-etch shadowing was performed on suspensions of partially purified tubules and whole bacteria. This procedure revealed the presence of ridges and grooves defining a set of long-pitch left-handed helical ridges. The long-pitch helices represented chains of acyltransferase dimers. Tubules observed within the cell were often closely packed, with an apparent alignment of grooves and ridges in adjacent tubules. Fracture planes passing through the tubules indicated the presence of a bilayer structure, with some portion of the enzyme being associated with the membrane. The major portion of the enzyme extended from the hydrophilic surface, forming a large globular structure that, in favorable views, displayed a central cavity facing the cytoplasm. Computer analysis of shadowed tubules revealed that the left-handed helices were six stranded, with a pitch of 1,050 A (105.0 nm) and a spacing of 75 A (7.5 nm) between acyltransferase dimers along the chains. Analysis of the predicted secondary structure failed to reveal obvious transmembrane segments, suggesting that very little of the protein was inserted into the bilayer.

Studies on reconstituted partially purified glycerophosphate acyltransferase from Escherichia coli

Solubilized glycerophosphate acyltransferase from Escherichia coli was reconstituted in small unilamellar vesicles consisting of phosphatidylcholine/phosphatidylglycerol in a molar ratio of 4:1. Glycerol 3-phosphate, trapped inside these vesicles, cannot be acylated by the enzyme upon addition of extra-vesicular palmitoyl-CoA. Thus, substrate-binding sites and active sites are asymmetrically oriented in the model membrane. When up to 10 mol/100 mol lysophosphatidic acid was incorporated in the vesicles a decrease in glycerophosphate acyltransferase activity is observed at amounts exceeding 1 mol% lysophosphatidate. Similar experiments, using lysophosphatidylcholine and phosphatidic acid, suggest the decrease to result from an increase in negative surface charge. Reconstituted glycerophosphate acyltransferase exhibits a preference for palmitoyl-CoA over oleoyl-CoA. This preference increases considerably at elevated temperatures. The glycerophosphate acyltransferase could, therefore, participate in the temperature-dependent changes in the fatty acid composition of the phospholipids in E. coli.

Facilitated utilization of endogenously synthesized lysophosphatidic acid by 1-acylglycerophosphate acyltransferase from Escherichia coli

Complete separation of glycerophosphate acyltransferase and 1-acylglycerophosphate acyltransferase from Escherichia coli was obtained by sequential extraction with Triton X-100. Solubilized glycerophosphate acyltransferase was reconstituted by the cholate dispersion and gel filtration method in small unilamellar vesicles. 1-Acylglycerophosphate acyltransferase could not be solubilized from the membranes and was used in endogenous membrane fragments after detergent removal. Mixing of the two preparations and subsequent incubation in the presence of glycerol 3-phosphate, palmitoyl-CoA and oleoyl-CoA resulted in the efficient synthesis of phosphatidic acid. Inclusion of exogenous lysophosphatitic acid in the assay medium resulted in a dilution of the newly synthesized lysophosphatidate. By contrast, the synthesis of phosphatidic acid from glycerol 3-phosphate by the acyltransferases present in native membrane vesicles was barely influenced by the presence of exogenous lysophosphatidic acid. When comparing the utilization of membrane-associated 14C-labeled and newly generated 3H-labeled lysophosphatidic acid, the latter appeared to be the preferred substrate. These results indicate that lysophosphatidic acid, synthesized by glycerophosphate acyltransferase, is utilized by 1-acylglycerophosphate acyltransferase without prior mixing with the total membrane-associated pool of lysophosphatidic acid, and suggest a close proximity of the two enzymes in native E. coli membranes. This property of the acyltransferases is lost upon separation and reconstitution of enzyme activities.