Antigenic structure of calmodulin: production and characterization of antisera specific for plant calmodulins or Ca2+-replete vs. Ca2+-free calmodulins.

Calmodulin was purified from bovine testis and soybean seed, using affinity chromatography on (6-amino-hexyl)-5-chloro-1-naphthylene sulfonamide (W7)-Sepharose. The simple preparation of this affinity medium is presented. Although bovine and soybean calmodulins share affinity for W7-Sepharose, share biological activity, and have similar amino acid compositions, antisera raised against soybean calmodulin recognize animal calmodulins with 20,000-fold less avidity than they do plant calmodulins. We have also raised antisera which recognize only Ca2+-calmodulin; they are incapable of recognizing the Ca2+-free conformation of calmodulin. Two classes of such antisera were obtained. One class exhibited half-maximal binding of [125I]-calmodulin with 0.5 microM Ca2+ while the other required 2.5 microM ionic Ca2+ for the same degree of antigen binding. These studies together indicate that plant calmodulins possess an epitope not found on animal calmodulins, and that the conformational alterations calmodulin undergoes upon binding Ca2+ can be exploited to raise antisera against those conformations.