DIFFERENTIAL LOCALIZATION OF MYOSIN-II ISOZYMES IN HUMAN CULTURED-CELLS AND BLOOD-CELLS

We used purified polyclonal antibodies to human cytoplasmic myosin-IIA and myosin-IIB directly labeled with fluorescent dyes to localize the se myosin-II isozymes in HeLa cells, melanoma cells and blood cells, B oth antibodies react strongly with myosin-II isozymes in HeLa cells, m elanoma cells and blood eosinophils, but only anti-myosin-IIA antibodi es stain platelets, lymphocytes, neutrophils and monocytes in smears o f human blood, Both antibodies stain small spots along the stress fibe rs of interphase HeLa cells and melanoma cells, but double staining re vealed that the detailed distributions of myosin-IIA and myosin-IIB di ffer, A low concentration of diffuse myosin-IIB is present in the cort ex, both in lamellar regions around the periphery of the cell and over the free surface, Myosin-IIB is also concentrated in spots along peri nuclear stress fibers, Myosin-IIA is absent from the cortex but is con centrated in spots along stress fibers located near the basal surface of cultured cells, This population of peripheral stress fibers is high ly enriched in myosin-IIA relative to myosin-IIB, but both are found t ogether in centrally located stress fibers, In prophase and metaphase both isozymes are concentrated in the cortex in small spots less than 04 mu m in size, similar to those in stress fibers, As the chromosomes begin the separate at anaphase, most of the myosin-II spots become co ncentrated in the outer 0.7 mu m of the equatorial cortex in 100% of c ells, This concentration of myosin-II isozymes in the cleavage furrow is maintained until the daughter cells separate, The superimposition o f these small spots concentrated in the cleavage furrow produces the i ntense, uniform staining observed in conventional micrographs of whole cells,