A Ca2+-dependent modulator protein has been isolated from BHK-21 cells. The purification requires heat treatment, ion-exchange chromatography, and gel filtration. The protein appears homogeneous on sodium dodecyl sulfate-polyacrylamide and isoelectric focusing gels. The protein comigrates with purified smooth muscle and brain modulators. BHK-21 modulator is characterized by a high content of aspartic and glutamic acids and by a high phenylalanine/ tyrosine ratio. It lacks both cysteine and tryptophan. The protein is effective in activating brain-modulator-deficient phosphodiesterase. It can also be used in assay systems to generate Ca2+-sensitive actin activation of both BHK-21 and smooth muscle myosins. Therefore, it is proposed that the BHK-21 modulator protein is a component of the Ca2+-dependent mechanism involved in the regulation of actin-myosin interactions in BHK-21 cells.
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