Characterization of the Human Calmodulin-Like Protein Expressed in Escherichia Coli

The protein-coding region of an intronless human calmodulin-like gene [Koller, M., Strehler, E.E. (1988) FEBS Lett. 239, 121-128] has been inserted into a pKK233-2 expression vector, and the 148-residue, M_x = 16 800 human protein was purified to apparent homogeneity by phenyl-Sepharose affinity chromatography from cultures of Escherichia coli JM105 transformed with the recombinant vector. Several milligrams of the purified protein were obtained from 1 L of bacterial culture. A number of properties of human CLP were compared to those of bacterially expressed human calmodulin (CaM) and of bovine brain CaM. CLP showed a characteristic Ca²⁺-dependent electrophoretic mobility shift on SDS-polyacrylamide gels, although the magnitude of this shift was smaller than that observed with CaM. CLP was able to activate the 3',5'-cyclic nucleotide phosphodiesterase to the same K_ma, as normal CaM, albeit with a 7-fold higher K_act. In contrast, the erythrocyte plasma membrane Ca²⁺-ATPase could only be stimulated to 62% of its maximal CaM-dependent activity by CLP. CLP was found to contain four Ca²⁺-binding sites with a mean affinity constant of 10⁵ M^_1, a value about 10-fold lower than that for CaM under comparable conditions. The highly tissue-specifically-expressed CLP represents a novel human Ca²⁺-binding protein showing characteristics of a CaM isoform.