Disulfide-linked dimer of oncomodulin: comparison to calmodulin.

Oncomodulin, an oncofetal Ca2+-binding protein, contains a single Cys residue in position 18 of its primary structure. The reactivity of the Cys-18 thiol has been probed with 5,5'-dithiobis(2-nitrobenzoate) (NbS2). The kinetics of the reaction indicate that the thiol group is approximately 10-fold more reactive in the presence of Ca2+ than in its absence. Evidence presented here shows that oncomodulin can dimerize by intermolecular disulfide formation via the Cys-18 thiol. The kinetics of dimer formation indicate that the second-order rate constant for this reaction is approximately 6-fold higher than that observed for the reaction of the Cys-18 thiol with NbS2, possibly indicating that intermolecular electrostatic interactions precede disulfide formation. The disulfide-linked dimer of oncomodulin appears to be more similar to calmodulin than oncomodulin since the dimer displayed "calmodulin-like" affinity for the amphiphilic peptide melittin. In addition, oncomodulin dimer was shown to activate two calmodulin-dependent enzymes, cyclic nucleotide phosphodiesterase and calcineurin phosphatase, with the activity constants of 63 and 1 nM, respectively, indicating that these enzymes have different domain contact requirements for activation.