Characteristics and biosynthesis of membrane proteins of lipid bodies in the scutella of maize (Zea mays L.).

Storage lipid bodies, which are prominent organelles present in the storage tissues of most seeds, have not been subjected to intensive biochemical investigation. In the present studies the major proteins in lipid bodies isolated from eleven taxonomically diverse species were shown to be distinctly different, as revealed by SDS/polyacrylamide-gel electrophoresis. The lipid-body membrane of maize (Zea mays L.) contained three major proteins of low Mr (19,500, 18,000 and 16,500), and they were chosen for further study. They all had alkaline pI values and behaved as hydrophobic integral proteins, as shown by their resistance to solubilization after repeated washing, amino acid composition and partitioning in a Triton X-114 system. Labelling in vivo with [35S]methionine and translation in vitro using extracted RNA in a wheat-germ system showed that the proteins were synthesized during seed maturation and not germination. The proteins synthesized in vivo and in vitro exhibited no appreciable difference in their mobilities in two-dimensional gel electrophoresis (isoelectric focusing and molecular sieving). The most abundant protein, that of Mr 16,500, was shown to be synthesized predominantly, if not exclusively, by RNA derived from bound polyribosomes and not from free polyribosomes. The implication of the results on the biosynthesis of the lipid bodies is discussed.