title: Disulfide-linked dimer of oncomodulin: comparison to calmodulin.
abstract: 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.
authors.affiliation: Department of Chemistry and Biochemistry, University of Windsor, Ontario, Canada.
authors.forename: B
authors.initials: B
authors.lastname: Mutus
authors.forename: E J
authors.initials: EJ
authors.lastname: Palmer
authors.forename: J P
authors.initials: JP
authors.lastname: MacManus
mesh_terms: D000818:Animals; D002135:Calcium-Binding Proteins; D002147:Calmodulin; D003545:Cysteine; D004220:Disulfides; D008114:Liver Neoplasms, Experimental / Q000032:analysis; D046911:Macromolecular Substances; D008555:Melitten; D010084:Oxidation-Reduction; D011485:Protein Binding; D011487:Protein Conformation; D051381:Rats; D013329:Structure-Activity Relationship
chemical_list: D002135:Calcium-Binding Proteins; D002147:Calmodulin; D004220:Disulfides; D046911:Macromolecular Substances; C034733:oncomodulin; D008555:Melitten; D003545:Cysteine