EXCHANGEABLE APOLIPOPROTEINS OF INSECTS SHARE A COMMON STRUCTURAL MOTIF

Elucidation of the secondary structure of the exchangeable apolipoprot eins has been hindered by the difficulty in producing crystals suitabl e for X-ray spectrographic analyses. Consequently, in order to analyze potential structure-function relationships in the family of insect ex changeable apolipoproteins, apolipophorins-III (apoLps-III), two apoLp s-III cDNA clones, one from the pale verde beetle (Derobrachus geminat us) and one from the house cricket (Acheta domesticus), have been isol ated and sequenced. Multiple sequence alignments of the deduced protei n sequences with two previously reported apolipophorins-III from Mandu ca sexta and Locusta migratoria reveal low sequence identity, suggesti ng that these proteins are very old and are highly divergent. Computer -assisted predictions of protein structure and subsequent analyses, us ing the known secondary structure of Locusta migratoria apolipophorin- III as a control, in dicate that these insect proteins are composed of five amphipathic helices with characteristics similar to those of the helical domains of the mammalian exchangeable apolipoproteins. Thus, although insect and vertebrate exchangeable apolipoproteins share a co mmon function in assisting lipid transport, precise amino acid identit y is less important than the common structural feature of multiple amp hipathic helices. Moreover, because these proteins occur widely among insect species, even in those where flight is limited or absent, we hy pothesize that apolipophorin-III has a more generalized function in li pid metabolism than had been previously proposed.