Membranes of protein bodies. II. Detection and partial characterization of membrane glycoproteins

A phospholipase A2 is transiently synthesized during seed germination and localized to lipid bodies

A patatin-like protein is present in the storage tissue of cucumber seedlings during the stage of fat mobilization. The cucumber protein is a homologue of a glycoprotein which in potatoes accounts for most of the total protein content of tubers. Following preparation of a cucumber cDNA library representing the developmental stage of cotyledons of 1 day old germinating seeds we isolated and characterized a clone encoding a patatin-like protein. Antibodies raised against the protein expressed in bacteria were used for immunodetection in subcellular fractions of cucumber seedlings. It was shown that the patatin-like protein was virtually exclusively confined to lipid bodies. The protein expressed in bacteria was characterized in vitro by its esterase activity acting on monoacylglycerols and phospholipids. Detailed analysis using various forms of phosphatidyl choline as substrates demonstrated that the patatin-like protein is a phospholipase A2 acting on palmitoyl, linoleoyl and hydroperoxidized linoleoyl groups equally well. Studying the temporal and tissue-specific expression of patatin-like protein mRNA we showed its appearance exclusively during fat catabolism. As maximal amounts of the protein were found at an early stage of mobilization and confined to lipid bodies, we propose that the patatin-like hydrolase is involved in lipid body mobilization.

Membranes of protein bodies. I. Isolation from cotyledons of germinating cucumber seeds

Protein bodies were prepared from cotyledons of germinating seeds of cucumber (Cucumis sativus) in different ways: the organelles either obtained from protoplasts by lysis or from cotyledons by mechanical disintegration were separated on sucrose-density gradients. In addition, a non-aqueous procedure was employed to isolate protein bodies. Marker proteins indicative of membranes of other organelles were carefully assayed. By this means contaminations in the purified protein-body fractions could be ruled out. Isolated protein bodies were separated into crystalloids, matrix, and membranes. The membranes were purified and characterized according to their equilibrium density (rho = 1.20 kg/l) on sucrose gradients by flotation or sedimentation. Protein-body membranes labelled in the phospholipid moiety were prepared and analyzed after application of [methyl-14C] choline or [32P] phosphate in vivo.