Activation of Bordetella pertussis adenylate cyclase by the carboxyl-terminal tryptic fragment of calmodulin

Highly purified tryptic fragments of calmodulin were tested for their ability to stimulate adenylate cyclase activity of Bordetella pertussis spheroplast membranes and were compared to their activities on brain Ca2+/calmodulin-dependent cyclic nucleotide phosphodiesterase. The C-terminal fragment, consisting of residues 78-148, was a full agonist for the cyclase with 0.1-0.15 the potency of calmodulin but did not stimulate phosphodiesterase. Fragments 1-77, 1-90, and 107-148 stimulated adenylate cyclase (and not phosphodiesterase) at low potency; this was not due to calmodulin contamination, but contamination by fragment 78-148 could not be excluded with certainty. An adduct of norchlorpromazine isothiocyanate and calmodulin showed full agonist activity for adenylate cyclase at 0.01-0.02 the potency of calmodulin. Stimulation of adenylate cyclase by a number of the fragments occurred in the absence of Ca2+, but stimulator potency was enhanced 20-60-fold in its presence. The similarity of Ca2+ requirements of fragment 78-148 and calmodulin suggests that occupancy of the two C-terminal Ca2+ binding sites of calmodulin accounts for most of the Ca2+ enhancement of calmodulin stimulation of adenylate cyclase.