Switchover of protein synthesis occurs during the wandering stage in the labial glands of the sweet potato hornworm, Agrius convolvuli [Sphingidae]

Labial glands of the sweet potato hornworm Agrius convolvuli are homologous organs with silk glands of the silkworm Bombyx mori. Previously, we indicated that the lumen proteins from the glands were different in molecular size between the 5th instar feeding stage and the wandering stage. To explain why this change occurs, we examined in vivo and in vitro incorporation of [sup(35)S] methionine in these proteins, and also examined the amino acid composition of typical proteins from each developmental stage. By SDS-PAGE, three major proteins (67, 70, and 116 kDa) were detected in the lumen of the gland during the 5th instar. These proteins disappeared within 12 hr after one set of the wandering stage, followed by the appearance of two high molecular mass proteins (180 and 220 kDa). Sealing the spinneret of the gland had no effect on the change observed in protein composition, and labeling experiments revealed that [sup(35)S] methionine was incorporated in each stage specific protein. In addition, the amino acid composition of the two major proteins at each stage (67 and 180 kDa) was specific to the respective stage. These results suggest that the switchover of protein synthesis occurs at the transcriptional level, accompanied by a behavioral change from feeding to wandering.