The role of diacylglycerol-carrying lipoprotein I in lipid transport during insect vitellogenesis.

A diacylglycerol-carrying lipoprotein was isolated from mature eggs of the silkworm, Philosamia cynthia and compared, for physiochemical properties, with the major diacylglycerol-carrying lipoprotein I (LP-I) of hemolymph. The two molecules are identical an electrophoretic mobility, structural configuration as revealed by electron microscopy, and amino acid composition. In addition mannose was detected in the lipid-free protein moiety, thus enabling classification of the molecules as glycoproteins. The molecules differ in lipid content with egg-LP-I containing only 3.6% of the diacylglycerol content of hemolymph LP-I and the phospholipid and cholesterol components also showing a marked reduction. Analysis of the LP-I and vitellogenin (another diacylglycerol-carrying lipoprotein; LP-II) concentrations of mature eggs indicates that the amounts of the two glycolipoproteins in eggs are insufficient to account for the total acylglycerol content of the eggs. The results suggest that LP-I functions as a true carrier-protein serving to transport diacylglycerol from the fat body to the ovary. This proposal is supported by the observation that LP-I isolated from the egg retains the physiological capacity of hemolymph-LPi to take up diacylglycerol from fat body. Thus it is suggested that LP-I is the major source of lipid for vitellogenesis, whereas vitellogenin is the primary source of protein.