Isolation and some structural and functional properties of macrophage tropomyosin

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Abstract
Tropomyosin purified from rabbit lung macrophages is very similar in structure to other nonmuscle cell tropomyosins. Reduced and denatured, the protein has 2 polypeptides which migrate during electrophoresis in sodium dodecyl sulfate on polyacrylamide gels, with slightly different mobilities corresponding to apparent MW of .apprx. 30,000. Following cross-linking by air oxidation in the presence of CuCl2, electrophoresis under nonreducing conditions reveals a single polypeptide of MW 60,000. Macrophage tropomyosin has an isoelectric point of 4.6 and an amino acid composition similar to other tropomyosins. It contains 1 cysteine residue/chain. In the EM, macrophage tropomyosin molecules rotary-shadowed with Pt and C are slender, straight rods, 33 nm in length. Macrophage tropomyosin paracrystals grown in high Mg concentrations have an axial periodicity of 34 nm. On the basis of yields from purification and from 2-dimensional electrophoretic analyses of macrophage extracts, tropomyosin comprises < 0.2% of the total macrophage protein, a molar ratio of .apprx. 1 tropomyosin molecule to 75 actin monomers in the cell. Macrophage tropomyosin binds to actin filaments. Macrophage, skeletal muscle and other nonmuscle cell tropomyosins inhibit the fragmentation of actin filaments by the Ca2+-gelsolin complex. Tropomyosin may have a role in stabilizing actin filaments in vivo.

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