Structural Consequences of Genetically Engineered Saturation of the Fatty Acids of Phosphatidylglycerol in Tobacco Thylakoid Membranes. An FTIR Study

The role of phosphatidylglycerol (PG) in protein-lipid interactions and membrane dynamics has been studied in the thylakoids of wild type and manipulated tobacco plants transformed with complementary DNAs for glycerol-3-phosphate acyltransferases (GPATs) from squash and Arabidopsis. The expression of the foreign enzymes resulted in the level of saturation of the PG molecules being higher in the squash and lower in the Arabidopsis transformants, as compared with the level in wild-type tobacco. For the analysis of fatty acyl chain dynamics in the thylakoid membranes, the nu(sym)CH(2) vibration bands of the infrared specta were decomposed into two components, corresponding to ordered and disordered fatty acyl chain segments. With this approach, it was shown that in squash GPAT-transformed tobacco thylakoids a rigid lipid domain exists below 25 degrees C. Above 25 degrees C, the dynamics of all thylakoid membranes were very similar, regardless of the manipulations. PG seems to tune the dynamics at the protein-lipid interface rather than to affect the structure of the proteins directly. Above 50 degrees C, the frequencies of the disordered nu(sym)CH(2) component bands were decreased. This lipid-related phenomenon correlated with protein denaturing. It is demonstrated that the protein aggregation appearing upon heat denaturing changes the conformational distribution of the disordered lipid population. The data also reveal that the protein stability does not depend on the fatty acid composition of the PG molecules; other lipids should provide the environment governing the protein stability in the thylakoid membrane. This is the first such detailed analysis of the infrared spectra of biological membranes that permits a differentiation between structurally different lipid populations within a membrane.