Tropomyosin fractions

Sea urchin egg tropomyosin has been isolated from an immunoprecipitate formed between anti-(lantern muscle tropomyosin) antibody and a sea urchin egg tropomyosin fraction. The antibody-antigen complex was heated to denature the antibody and the components were then separated. 

The contractile proteins of the myofibril include three troponin regulatory proteins. The troponin complex includes three protein subunits, troponin C (the calcium-binding component), troponin I (the inhibitory component), and troponin T (the tropomyosin-binding component). The subunits exist in a number of isoforms. The distribution of these isoforms varies between cardiac muscle and slow- and fast-twitch skeletal muscle. Only two major isoforms of troponin C are found in human heart and skeletal muscle. These are characteristic of slow- and fast-twitch skeletal muscle. The heart isoform is identical with the slow-twitch skeletal muscle isoform. Isoforms of cardiac-specific troponin T (cTnT) and cTnl also have been identified and are the products of unique genes. All cardiac troponins are localized primarily in the myofibrils (94%-97%), with a smaller cytoplasm fraction (3%-6%). 

Tropomyosin extracted at ij. 1, soluble at low ionic strength. When the mixture of Fig. 15 is diluted to p 0.05, tropomyosin can be isolated from the supernatant by the usual procedure. In this case, however, the acidification has been carried out only as far as pH 5.5 instead of 4.6. The fraction examined thus corresponds to a part of the tropomyosin soluble at low ionic strength present in the extracts. Its electrophoretic picture is similar to that of Fig. 13. When the preparation is precipitated at pH 4.6 and at relatively high salt concentration (p 1), it is profoundly transformed the fast electrophoretic component which corresponds to Bailey s tropomyosin increases notably at the expense of the slow one. The same transformation is observed by ultracentrifugation components sedimenting at rates varying between those of tropomyosin and nucleotropomyosin become less important while the slow tropomyosin peak increases (Fig. 30). As the nucleic acid present in the preparations does not precipitate with tropomyosin at p 1 and pH 4.6, the question arises if this transformation is not due to its removal. As the amounts present are. 

A stepwise degradation of the tropomyosin complex thus occurs in the course of the extraction, leading to a progressive increase of solubility and allowing the identification of more or less different fractions. The particle described by Bailey (1948) corresponds to the final step of this transformation. Tropomyosin may actually exist in fish muscle as a ribonucleoprotein but as the ultracentrifugal behavior typical of nucleotropomyosin is ob-... 

As originally pointed out by Cohen and Szent-Gyorgyi (12), the soluble fibrous proteins which possess the a-type wide-angle x-ray diffraction diagram, with the exception of fibrinogen, show complex rotatory dispersion similar to that of the synthetic polypeptides. Light meromyosin Fraction I (LMM Fr. I), tropomyosin, and paramyosin all have over 90% helix (12)... 

The tropomyosin content in the case of rabbit muscle is about 6 % of the total protein (Bailey, 1948), and it is not improbable that in the scheme of fractionation given above it is included in the L-myosin value. 

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