Calcium-independent stimulation of Bordetella pertussis adenylate cyclase by calmodulin

Abstract

Bordetella pertussis produces an extracellular adenylate cyclase activity that is present in the culture medium of exponentially growing cells. We have determined that calmodulin (CaM) stimulates this enzyme both in the presence and in the absence of free Ca2+. In the presence of 90 micron Ca2+ half-maximal stimulation of the enzyme occurred at 95 pM calmodulin. Comparable levels of calmodulin stimulation were observed when free Ca2+ levels were minimized by using EGTA-containing buffers. However, the concentration of CaM required for half-maximal stimulation of B. pertussis adenylate cyclase in the presence of 1 nM free Ca2+ was 24 nM. The apparent affinity of the enzyme for calmodulin was also significantly enhanced by Mn2+. In addition, troponin I inhibited calmodulin stimulation of the bacterial adenylate cyclase. Photoaffinity cross-linking experiments using azido[125I]calmodulin and B. pertussis adenylate cyclase revealed only one major cross-linked product having a molecular weight of 97000. It is proposed that the catalytic subunit of the calmodulin-sensitive adenylate cyclase is 77000.