Molecular characteristics of insect vitellogenins

Vitellogenins (Vgs) are precursors of the major egg storage protein, vitellin (Vn), in many oviparous animals. Insects Vgs are large molecules ( approximately 200-kD) synthesized in the fat body in a process that involves substantial structural modifications (e.g., glycosylation, lipidation, phosphorylation, and proteolytic cleavage, etc.) of the nascent protein prior to its secretion and transport to the ovaries. However, the extent to which Vgs are processed in the fat body varies greatly among different insect groups. We provide evidence by cloning and peptide mapping of four Vg molecules from two cockroach species (Periplaneta americana and Leucophaea maderae) that, in hemimetabolous insects, the pro-Vg is cleaved into several polypeptides (ranging from 50-to 180-kD), unlike the holometabolans where the Vg precursor is cleaved into two polypeptides (one large and one small). An exception is the Vg of Apocrita (higher Hymenoptera) where the Vg gene product remains uncleaved. The yolk proteins (YPs) of higher Diptera (such as Drosophila) form a different family of proteins and are also not cleaved. So far, Vgs have been sequenced from 25 insect species; 9 of them belong to Hemimetabola and 16 to Holometabola. Alignment of the coding sequences revealed that some features, like the GL/ICG motif, cysteine residues, and a DGXR motif upstream of the GLI/CG motif, were highly conserved near the carboxy terminal of all insect Vgs. Moreover, a consensus RXXR cleavage sequence motif exists at the N-terminus of all sequences outside the Apocrita except for Lymantria dispar where it exists at the C-terminus. Phylogenetic analysis using 31 Vg sequences from 25 insect species reflects, in general, the current phylogenies of insects, suggesting that Vgs are still phylogenetically bound, although a divergence exists among them.