Occurrence, quaternary structure and function of G protein subunits in an insect endocrine gland

The occurrence, structure and function of the alpha and beta subunits of GTP-binding proteins (G proteins) were investigated in the Manduca sexta prothoracic gland, a tissue which possesses a hormonally regulated adenylate cyclase. Subunit-specific antibodies were utilized in immunoblotting studies of tissue from Manduca prothoracic glands, brain, eyes and antennae, and compared to the substrates present in the heads of Drosophila, as well as in a mammalian cell line. All Manduca tissues examined showed putative G beta subunits of 37 and 38 kDa, an unidentified alpha subunit of 41 kDa, in addition to an eye specific alpha subunit of 42 kDa. Manduca tissues also produced putative Gs alpha subunits of 48 and 51 kDa which were coupled to prothoracic gland adenylate cyclase as demonstrated by immunoprecipitation. Prothoracic gland G proteins have a definite and limited quaternary structure, consistent with a heterotrimeric model, as demonstrated by crosslinking of prothoracic gland membrane preparations followed by immunoblotting. These studies also yielded data on relative titers of alpha subunits, and suggest that Gs alpha is present in lower amounts than other alpha subunits. The G protein subunits studied in the prothoracic gland appear strikingly similar in molecular weight, function and structure to their mammalian counterparts.