Characteristics of apolipophorin-III of the southwestern corn borer, Diatraea grandiosella

Apolipophorin-III was isolated from the lipophorin-free fraction of larval plasma of the southwestern corn borer, Diatraea grandiosella, because significant amounts of apolipophorin-III were found to be present in the hemolymph not associated with lipophorin. Apolipophorin-III, purified using ammonium sulfate precipitation, cation exchange chromatography, and gel filtration, was shown to be a nonglycosylated polypeptide with 17 kDa mol. wt, as determined by SDS-PAGE and silver staining. The amino acid composition of apolipophorin-III showed similarities to published compositions of apolipophorin-III isolated from other insects. The N-terminal sequence of apolipophorin-III (DAPSTTPPQDXEKKAAEFQKTFTEQXNQLANK), is highly homologous to that of apolipophorin-III of Manduca sexta. Antiserum raised against purified apolipophorin-III was used to demonstrate an immunochemical identity between the isolated apolipophorin-III and that associated with lipophorin. This antiserum cross-reacted with apolipophorin-III of M. sexta, and antiserum raised against M. sexta apolipophorin-III cross-reacted with apolipophorin-III isolated from D. grandiosella, demonstrating an immunochemical relationship between the proteins, and providing confirmatory evidence for the identity of the isolated protein. These antisera did not react with the putative apolipophorin-III of the cricket, Acheta domesticus. Using immunoprecipitation by the apolipophorin-III antiserum of D. grandiosella, the synthesis and secretion of ³H]apolipophorin-III by the fat body in vitro was shown to be maximal in 13–15 day-old larvae, with a transit time of ca 23 min.