Differential staining of actin in metaphase spindles with 7-nitrobenz-2-oxa-1,3-diazole-phallacidin and fluorescent DNase: is actin involved in chromosomal movement?

The distribution and polymerization state of actin in metaphase rat kangaroo cells was studied by fluorescence microscopy. Formaldehyde-fixed, acetone-extracted cells were labeled with either of two types of actin probes. The first, 7-nitrobenz-2-oxa-1,3-diazole-phallacidin, has high affinity for F actin and does not bind monomeric G actin. The second was a conjugate of DNase I labeled with either tetramethylrhodamine or fluorescein. DNase binds with high affinity to G actin and with lesser affinity to F actin. The polymerization state of actin was deduced by comparing the fluorescence distribution of the phallacidin derivative with that of the fluorescent DNase. The results indicate that the pole-to-chromosome region of the metaphase spindle contains G actin but little if any conventional F actin. F actin is found concentrated in a diffuse distribution outside the spindle region in metaphase cells and returns to the interzone area between the chromosomes by early telophase. These results exclude spindle models for chromosomal movement that require more than about five F actin filaments per chromosome, support the hypothesis that F actin is involved in force generation for cell cleavage, and are not inconsistent with the possibility that actin outside the spindle may be involved in chromosomal movement.