Purification and some properties of porcine kidney tropomyosin

A tropomyosin has been purified from porcine kidney and its properties compared with those of rabbit skeletal muscle tropomyosin. Kidney tropomyosin was separated from contaminating vascular smooth muscle tropomyosin by hydroxylapatite chromatography. Kidney tropomyosin resembles tropomyosin from other non-muscle cells in regard to subunit size, mobility on SDS-polyacrylamide gels in the presence and absence of 6 M urea, amino acid composition and morphology. The binding of tropomyosin to F-actin is strongly dependent on the Mg2+ concentration. With kidney tropomyosin, binding begins at 1 mM Mg2+ and is complete at about 4-5 mM, while with muscle tropomyosin, binding is initiated at 1 mM Mg2+ and reaches saturation at 2-3 mM Mg2+. Both kidney and muscle tropomyosins bind to actin in a similar ratio of 1 tropomyosin/6-7 actin monomers at saturation. Both kidney and skeletal muscle tropomyosins prevent "severing" of F-actin filaments induced by gelsolin/villin-like protein purified from kidney. These results suggest that, in non-muscle cells, tropomyosin may protect microfilaments from Ca2+-dependent solation at the site where they may interact with myosin.