Tropomyosin heterogeneity in human cells.

Tropomyosin preparations from human platelets, human peripheral blood leukocytes from normal individuals and from a patient with chronic lymphocytic leukemia, human lymphoblastoid cells (GM607), human epithelial cells, and human skin fibroblasts have all been found to contain more than one protein when analyzed by two-dimensional gel electrophoresis. Although the lymphoid cell preparations consistently contain two proteins of almost identical molecular weight (Mr = 30,000), the platelet, epithelial cell, and fibroblast preparations contain two or more major proteins with molecular weights between 31,000 and 36,000, in addition to a major protein at 30,000. All of these proteins have characteristics in common with tropomyosin including slightly acidic isoelectric point (approximately pH 4), stability to heat and organic solvents, association with the cytoskeleton, and reactivity with antibody against skeletal muscle tropomyosin. The nonmuscle tropomyosin-like proteins were compared with tropomysins from human skeletal, cardiac, and smooth muscle by peptide mapping after partial proteolysis. The results showed one of the non-muscle proteins to be identical to the major smooth muscle tropomyosin in human uterus (myometrium) and another to be similar but not identical to skeletal muscle alpha-tropomyosin. The remainder of the proteins with tropomyosin characteristics was unique to non-muscle cells. In all, nine distinct human proteins with characteristics of tropomyosin are described. Charge variants of two of these proteins have been described previously.