Microtubule‐Associated Cyclic AMP‐Dependent Protein Kinase in Drosophila melanogaster

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Abstract
Microtubules were prepared from head extracts of the adult fruit fly, Drosophila melanogaster, by one‐step, taxol‐assisted polymerization. The microtubular fraction displayed cyclic AMP‐dependent protein kinase (protein kinase A) activity, as witnessed by endogenous protein phosphorylation and by protein kinase assay. Microtubule‐bound protein kinase A amounts to 4–5% of total soluble kinase activity, which is almost an order of magnitude less than in mammals. The high‐molecular‐weight microtu‐bule‐associated protein‐2 (MAP‐2), the main binding species for protein kinase A in mammalian brain microtubules, is not detectable in the fly system by protein staining and immunoblotting with anti‐pig MAP‐2 serum, as well as by hybridization of fly DNA with a cDNA probe for human MAP‐2. Cyclic AMP removes a major part of the regulatory (R) subunit of the enzyme from Drosophila microtubules, as demonstrated by enzyme assay, autophosphoryla‐tion of R subunit, and quantitating cyclic AMP binding sites. It is proposed that permanently elevated cyclic AMP levels may elute protein kinase A from crucial intracellular binding sites, thereby interfering with signal transduction.