We have studied the distribution of myosin molecules in human cells using myosin-specific antibody coupled with fluorescent dyes. Rabbits were immunized with platelet myosin or myosin rod. They produced antisera which precipitated only myosin among all the components in crude platelet extracts. From these antisera we isolated immunoglobulin-G (IgG) and conjugated it with tetramethylrhodamine or fluorescein. We separated IgG with 2-5 fluorochromes per molecule from both under- and over-conjugated IgG by ion exchange chromatography and used it to stain acetone-treated cells. The following controls established the specificity of the staining patterns: (a) staining with labeled preimmune IgG; (b) staining with labeled immune IgG adsorbed with purified myosin; (c) staining with labeled immune IgG mixed with either unlabeled preimmune or immune serum; and (d) staining with labeled antibody purified by affinity chromatography. In blood smears, only the cytoplasm of platelets and leukocytes stained. In spread Enson and HeLa cells, stress fibers stained strongly in closely spaced 0.5 mum spots. The cytoplasm stained uniformly in those cells presumed to be motile before acetone treatment. In dividing HeLa cells there was a high concentration of myosin-specific staining in the vicinity of the contractole ring and in the mitotic spindle, especially the region between the chromosomes and the poles. We detected no staining of erythrocytes, or nuclei of leukocytes and cultured cells, or the surface of platelets and cultured cells.

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