The amino acid sequence of the troponin C-like protein (modulator protein) from bovine uterus

Abstract

Recent studies have shown that a troponin C-like protein is present in vertebrate smooth muscle [l-5] . This protein can form Ca”-dependent complexes with troponin I from fast skeletal muscle in a similar manner to those obtained with skeletal muscle troponin C [2,3] . It can be isolated from rabbit uterus using an affinity chromatographic procedure which depends on this property of complexing with skeletal muscle troponin I [l] but amore convenient, relatively simple, method of isolating the protein from smooth muscle and other tissues in good yields has recently been described [3]. The troponin C-like protein from smooth muscle possesses other properties similar to those of skeletal muscle troponin C. These are the abilities to inhibit the phosphorylation of troponin I by 3’,5’-cyclic AMP-dependent protein kinase and to neutralise the inhibitory activity of skeletal troponin I on the Mg2+-stimulated ATPase of desensitized skeletal actomyosin [2] . Unlike skeletal muscle troponin C, however, the smooth muscle protein activates cyclic nucleotide phosphodiesterase and contains the unusual amino acid, trimethyl lysine [3] . These latter properties are a feature of the Ca2+binding protein of bovine brain, the brain modulator protein, which Amphlett et al. [6] have demonstrated can substitute in the skeletal troponin complex in the place of troponin C and will restore full calcium sensitivity to desensitized actomyosin in the presence of troponin I and tropomyosin alone. Cheung et al. [7] have shown that a protein similar to the brain modulator protein is present in many of the tissues of mammals, and Waisman et al. [8] have