Exchangeable apolipoproteins of insects share a common structural motif

Elucidation of the secondary structure of the exchangeable apolipoproteins has been hindered by the difficulty in producing crystals suitable for X-ray spectrographic analyses. Consequently, in order to analyze potential structure-function relationships in the family of insect exchangeable apolipoproteins, apolipophorins-III (apoLps-III), two apoLps-III cDNA clones, one from the palo verde beetle (Derobrachus geminatus) and one from the house cricket (Acheta domesticus), have been isolated and sequenced. Multiple sequence alignments of the deduced protein sequences with two previously reported apolipophorins-III from Manduca sexta and Locusta migratoria reveal low sequence identity, suggesting that these proteins are very old and are highly divergent. Computer-assisted predictions of protein structure and subsequent analyses, using the known secondary structure of Locusta migratoria apolipophorin-III as a control, indicate 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 common function in assisting lipid transport, precise amino acid identity is less important than the common structural feature of multiple amphipathic helices. Moreover, because these proteins occur widely among insect species, even in those where flight is limited or absent, we hypothesize that apolipophorin-III has a more generalized function in lipid metabolism than had been previously proposed.