We have studied the distribution of myosin and tubulin molecules inside the same tissue culture cells by using two antibodies labeled with contrasting fluorochromes. Antimyosin raised against human platelet myosin was labeled with rhodamine. Antitubulin raised against sea urchin vinblastine-induced tubulin crystals was labeled with fluorescein. The two antibodies stained entirely different structures inside the same flat interphase cell: antimyosin bound to stress fibers and antitubulin bound to thin, wavy fibers thought to be individual microtubules. Compact interphase cells stained diffusely with both antibodies. From prophase through early anaphase both antibodies stained the mitotic spindle, although the fluorescence contrast between the spindle and the cytoplasm was much higher with antitubulin than with antimyosin. From anaphase through telophase, strong antimyosin staining occurred in the cleavage furrow, while antitubulin stained the region between the separated chromosomes. This study established the feasibility of high-resolution fluorescent antibody localization of pairs of motility proteins in the cytoplasm of single cells, an approach which will make it possible to map out the sites of the various contractile protein interactions in situ.

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