Lipophorin in developing adults of Hyalophora cecropia: Support of yolk formation and preparation for flight

Abstract

Lipophorin in Hyalophora cecropia changes from a high density form in diapausing pupal hemolymph (1.143 g/ml in males and 1.139 g/ml in females) to a predominantly low density form in resting adults (1.043 g/ml). The transition takes place primarily during adult eclosion. One week before eclosion, lipophorin undergoes changes in females that are correlated with yolk formation. It becomes 50% more concentrated than in males, its density drops to 1.124 g/ml (compared with 1.134 g/ml in males), it binds apolipophorin III with a stoichiometry of 1:1, and it is joined by small amounts of an adult-like low density lipophorin. Following ovariectomy, lipophorin differed only marginally from this, suggesting that a feedback mechanism prevents it from being encumbered with lipids beyond the level in females that are making yolk. During adsorptive endocytosis by the oocyte, 94% of the lipophorin that is deposited in the yolk is converted to a very high density form (1.232 g/ml), presumably due to the lipid unloading demonstrated earlier in two other lepidopterans. Apolipophorin III and some additional antigenic epitopes of the hemolymph lipophorins were undetectable in the unloaded yolk protein. Yolk contains in addition small amounts of free apoliphorin III and of a high density lipophorin similar to that of the hemolymph; both of these components occur at levels expected for proteins accumulated as solutes in the fluid phase of the oocyte's endocytotic vesicles. Several models are suggested to explain how the apolipophorin III that is bound to lipophorin in the hemolymph of vitellogenic females escapes deposition in the yolk.