Protein migration from transplanted nuclei in Amoeba proteus. I. The relation to the cell cycle and RNA migration, as...

Autoradiography has been used to examine the migration of proteins from a radioactively labelled amoeba nucleus following transplantation into an unlabelled homophasic amoeba. Nuclei were transferred at three times in the cell cycle coinciding with DNA synthesis (4 h post-division); a peak of RNA synthesis (25 h); and a relative lull in synthetic activity (43 h). Six amino acids were added individually to the culture medium to label the nuclear proteins. Migration of the proteins from the donor nucleus was found to be greatest following the transfer of [3H]aspartic acid-labelled nuclei and least with protein labelled with the basic amino acids. All amino acids exhibited the greatest extent of migration following the 25-h transfers, i.e., coinciding with a peak of RNA synthesis at 26-27.5 h. Actinomycin D (actD) inhibition of RNA synthesis reduced, but did not eliminate the extent of protein migration from the transplanted nucleus, thus indicating the existence of two classes of migratory proteins. Firstly, proteins, associated with RNA transport, which migrated mainly into the host cytoplasm. The second class migrated into the host nucleus from the transplanted nucleus, irrespective of RNa synthesis. The shuttling character of the latter class of proteins is consistent with a role of regulation of nuclear activity.