An insect lipoprotein hybrid helps to define the role of apolipophorin III

Insects transport lipid for flight in the form of diacylglycerol-rich low density lipoproteins (low density lipophorin (LDLp)). A hybrid LDLp has been produced in vitro by using Locusta migratoria fat body, locust high density lipophorin, locust adipokinetic hormone, and Manduca sexta apolipophorin III (apoLp-III). The hybrid is similar in size and density to locust LDLp, contains several molecules of M. sexta apoLp-III, and lacks locust apoLp-III, as shown by immunochemical methods. Under the same conditions an apoLp-III from Thasus acutangulus is poorly incorporated into the locust lipoprotein. The role of apoLp-III as a recognition signal/activator of flight muscle lipoprotein lipase was assayed with labeled hybrid LDLp produced in vitro using M. sexta apoLp-III radiolabeled with 35S. In addition, hydrolysis of diacylglycerol was determined with lipid-labeled hybrid LDLp produced in vitro using [U-14C]glycerol incorporated into the diacylglycerol moiety. In vitro incubations of the labeled hybrid LDLp with L. migratoria flight muscles show that the lipase efficiently utilizes hybrid LDLp as a substrate and demonstrate that the carbohydrate moiety of locust apoLp-III (which is lacking in the M. sexta protein) is not required for interaction with the lipase. It also suggests that specific antigenic determinants of L. migratoria apoLp-III are not required for lipase activation since M. sexta apoLp-III lacks immunological cross-reactivity with L. migratoria apoLp-III.