Nuclear actin: an apparent association with condensed chromatin.

In Amoeba proteus the concentration of actin is the same in the nucleus and cytoplasm. Certain characteristics, e.g., the ready loss of actin from isolated nuclei, suggest that the association with nuclei is normally not a tight one. Here we report, however, that when nuclei are isolated several hours after mitotic amebas are placed in actinomycin D (which allows normal progression of mitosis), the nuclei retain substantial amounts of actin. Since this finding correlates with other observations that the chromatin of such cells is extensively condensed, we suggest that a relationship may exist between actin and chromatin condensation.