Purification and Properties of the Components from Troponin

Cross-linking of troponin with dimethylimido esters

The topology of troponin, the calcium binding regulatory protein in muscle, has been studied by cross-linking with different length dimethylimido esters. The results show that the three components of troponin are close to each other and that the troponin-I and -T are preferentially cross-linked being 0.6 nm or less apart. The largest cross-linked product is a complex which corresponds in molecular weight to the native troponin complex of 1 mol of each of the three components. Cross-linked troponin has lost the ability to make the actomyosin ATPase calcium sensitive although it does bind to actin-tropomyosin and tropomyosin, and it binds calcium normally. No effect of calcium on cross-linking could be detected.

Regulation of muscle contraction: bindings of troponin and its components to actin and tropomyosin

The bindings of troponin components to actin and tropomyosin has been studied by cosedimentation with actin and affinity chromatography. It is shown that troponin binds to actin and tropomyosin in the presence and absence of calcium but the binding to actin is sensitive to ionic strength. Troponin-I + C binds to actin-tropomyosin in the absence of calcium but not to actin or tropomyosin alone. Troponin-I binds to actin and the binding is improved in the presence of tropomyosin even though troponin-I does not bind to tropomyosin alone. Troponin-C does not bind to actin or tropomyosin. The results suggest that the binding of troponin by actin is influenced by tropomyosin. A model of regulation by troponin is proposed.

Structural proteins of dogfish skeletal muscle

As part of a study on the evolutionary aspects of control mechanisms, a number of structural muscle components from the Pacific dogfish (Squalus acanthias) are described. These include troponin, tropomyosin, actin, and myosin. Troponin (mol wt 108.000) was resolved into its constitutive subunits, repeated by a 20,500 mol wt fragment which binds 2 mol of Ca2+/mol with a KDiss of 0.91 mum, and an inhibitory component of 30,000 and a 58,000 component which are necessary for the calcium sensitivity of actomyosin ATPase. Tropomyosin and actin share many properties with their counterparts from higher vertebrates. Proteins similar to parvalbumins, i.e., the low molecular weight calcium-binding proteins widely distributed in fish, amphibians, and mammalian muscle, could be generated from troponin and its calcium-binding subunit by limited proteolysis. The appearance of immunological cross-reactivity and other similar features suggested some identity, but differences in the amino acid analysis exclude the possiblity that parvalbumins occur as breakdown products of troponin. The close relationship between parvalbumins and the calcium-binding subunit brings additional evidence that these proteins have arisen through divergent evolution.