Developmental changes in cyclic AMP-dependent protein kinase associated with increased secretory capacity of Manduca sexta prothoracic glands.

In Manduca sexta, basal and PTTH-stimulated secretion of ecdysteroids by prothoracic glands in vitro increases from days 1 to 4 of the fifth larval stage. Glandular content of cAMP-dependent protein kinase was analyzed to determine if the enzyme changes in concert with increased secretory response. Photoaffinity labeling with [32P]8-N3 cAMP revealed a 55-kDa cAMP-binding protein characteristic of the regulatory subunit of type-II cAMP-dependent protein kinase (RII). It appears that RII is one of a limited number of cellular proteins that is phosphorylated in the presence of [gamma-35S]ATP; the thiophosphorylated protein and the photoaffinity-labeled regulatory subunit possess the same M(r) and pI, and thiophosphorylation is blocked by mammalian cAMP-dependent protein kinase inhibitor. From days 1 to 4 of the fifth instar, glandular content of RII increases in conjunction with increased ecdysteroid secretory capacity. Application of JH analog on day 1 significantly inhibits the observed increase in RII. Catalytic subunit activity does not change from days 1 to 4 of the fifth instar, nor does cellular content of a 34-kDa protein previously shown to be phosphorylated in response to PTTH. While it is unlikely that increased content of RII is solely responsible for enhanced ecdysteroid secretion by the prothoracic glands, it may serve as a convenient marker for investigating the mechanism by which steroidogenic capacity is regulated.