Identification of an Arylphorin-type Storage Protein in the Sweet Potato Hornworm, Agrius convolvuli

A major hemolymph protein (Mr480,000) in the larvae of the sweet potato hornworm, Agrius convolvuli, was purified and characterized. This protein was isolated with a high yield from the hemolymph of day 3 fifth final instar larvae by ammonium sulfate precipitation and Phenyl-Sepharose and Q-Sepharose column chromatographies. The protein has two subunits, an Mr84,000 subunit (α) and an Mr80,000 subunit (β), and the native protein was composed of a heterohexamer (α3β3). The two subunits have similar amino acid compositions, with high contents of aromatic amino acids (about 15% phenylalanine plus tyrosine) and low levels of methionine. The N-terminal amino acid sequences of both subunits showed high homologies with insect arylphorin-type storage proteins. The protein concentration in the hemolymph increased steeply from day 3 final instar larva and reached a maximum level of 42 mg/ml in females and 41 mg/ml in males among wandering larvae. The concentration in the hemolymph declined once during the larval–pupal transformation but remained high during the early–mid pupal period and almost disappeared after adult emergence. These quantitative changes were the same for males and females. Based on these characteristics, we identified the hemolymph protein as an arylphorin-type storage protein.