Primary structure and comparative sequence analysis of an insect apolipoprotein. Apolipophorin-III from Manduca sexta

The amino acid sequence of an insect apolipoprotein, apolipophorin-III from Manduca sexta, was determined by a combination of cDNA and protein sequencing. The mature hemolymph protein consists of 166 amino acids. The cDNA also encodes for an amino-terminal extension of 23 amino acids which is not represented in the mature hemolymph protein. The existence of a precursor protein was confirmed by in vitro translation of fat body mRNA. Computer-assisted comparative sequence analysis revealed the following points: 1) the protein is composed of tandemly repeating tetradecapeptide units with a high potential for forming amphiphilic helical structures. Compared to mammalian apolipoproteins the repeat units in the insect apolipoprotein show considerable length variability; 2) the sequence has a striking resemblance to several human apolipoproteins including apoE, AIV, AI, and CI. However, the homology seems to be entirely functional since, although the insect and mammalian apoproteins contain very similar types of amino acid residues, the actual degree of sequence identity is quite low. Whether the mammalian and insect apoproteins are derived from a common ancestral amphiphilic helix forming, lipid-binding protein, or arose by convergent evolution can not be determined at present. This represents the first complete amino acid sequence for an insect apolipoprotein.