Galactolipids of Thylakoid Pigment Protein Complexes Separated Electrophoretically from Thylakoids of Dunaliella salina Labeled with Radioactive Fatty Acids

Abstract

Experiments have been conducted to determine whether mono-and digalactosyldiacylglycerols (MGDG and DGDG) labeled by short term exposure of the green alga Dunaliella salina to exogenous radioactive lauric, myristic, or palmitic acids are preferentially recovered in one or more thylakoid protein complexes separated by electrophoresis on nondenaturing gels. A selective retention of certain lipid classes and molecular species by the major electrophoretically separable fractions, oligomeric form (LHCP-1) and monomeric form (LHCP-3) of hight-harvesting chlorophyll a/b protein complex, and major photosystem I protein complex (CPla), and nonprotein-bound lipids, was indicated. After 5 min of labeling with 3H-palmitate the specific radioactivities of MGDG and DGDG molecular species recovered from LHCP-3 and CPla were slightly higher and lower, respectively, than specific radioactivities of equivalent molecular species from LHCP-1 and nonprotein-bound lipid. However, the differences were smaller than those which have been observed between whole thylakoids and the more highly labeled chloroplast envelopes from briefly labeled cells. The processes for biosynthesizing and assembling polypep-tide, pigment, and acyl lipid components into a functional thylakoid membrane are under intensive study in many laboratories (13). Important unresolved questions remain with respect to the mechanism and location of complex acyl lipid formation within the chloroplast (16). Although convincing evidence has been obtained implicating the chloroplast envelope as the site for acyl chain esterification to glycerol 3-phosphate and subsequent addition of glycerol or galactose polar moieties, almost nothing is known regarding the desat-urase enzymes responsible for establishing the very high degree of unsaturation characterizing these lipids (6, 9). In previous work (4, 15), we have developed the green alga Dunaliella salina as a model system for the study of galacto-lipid metabolism in the chloroplast. Data regarding the sequence of desaturation steps were obtained, but the site(s) of fatty acid desaturase action within the organelle was not '