Isolation and characterization of a lipoprotein receptor from the fat body of an insect, Manduca sexta.

A lipoprotein receptor has been purified from the fat body of Manduca sexta larvae. The purification involves solubilization of membrane proteins in detergent, DEAE-, and hydroxyapatite chromatography, affinity chromatography on a concanavalin A column, and affinity chromatography on a lipoprotein-Sepharose column. An overall purification of 220-fold from the solubilized membranes was achieved. The receptor has an apparent molecular mass of 120 kDa. The receptor has an absolute requirement for Ca2+ and is inhibited by Suramin. The pH optimum of the receptor is 6.5, which is near the pH of the hemolymph. Binding data indicate a single high affinity binding site with a Kd = 4.1 +/- 0.19 x 10(-8) M as measured with the lipoprotein isolated from larval hemolymph. The major neutral lipid carried by insect lipoproteins is diacylglycerol, and it was shown that the affinity of the receptor for lipoprotein ligands correlates with their diacylglycerol content. It is proposed that the decrease in affinity of the receptor for lipoproteins depleted of diacylglycerol plays a key role in facilitating the transport of diacylglycerol from the midgut to the fat body during the larval feeding period. The insect receptor has some properties which are similar to those of vertebrate lipoprotein receptors, viz. molecular weight, requirement for Ca2+, and inhibition by Suramin. However, the insect receptor does not bind human low density lipoprotein.