Photolabeling of partially purified bovine brain adenylyl cyclase (AC I) with [γ 32P8-N-ATP led to incorporation of 32P into the 115 kDa catalyst. Further treatment with N-chlorosuccinimide, which cleaves proteins at tryptophan residues, yielded a 14 kDA 32P-labeled fragment. The latter was immunoprecipitated by antibody BBC1, recognizing the extreme C-terminus of AC I, but not by antibody BBC2, recognizing a more remote epitope. Further fragmentation of photolabeled AC I by the proteases Glu-C and Asp-N yielded 32P-labeled peptides corresponding to 2.9 kDa and 5.6 kDa fragments, which were not recognized by any of these antibodies. This narrows the ATP binding site down to a 25 amino acid sequence containing a general motif G(X0–7)KG(X0–4)L/M(X5–7)S/T present in all eukaryotic adenylyl cyclases so far cloned, but also in a variety of bacterial adenylyl cyclases.
