The accumulation of proteins in the Xenopus oocyte nucleus
The ability of proteins to accumulate in the nucleus has been studied by injecting nucleoplasmin and calf thymus histone H1 into the cytoplasm of Xenopus oocytes. Nucleoplasmin, the most abundant protein in the Xenopus oocyte nucleus is pentameric and proteolysis of the nuceloplasmin pentamer produces a relatively protease resistant 'core' molecule that cannot enter the nucleus after microinjection into the cytoplasm. The polypeptide domain ('lq tail') of each subunit removed by proteolysis was obtained as a discrete fragment and has the ability to accumulate in the nucleus. Partially cleaved pentameric molecules with a single intact sub unit can still accumulate in the nucleus. Therefore a polypeptide domain of nucleoplasmin has been found that is both necessary and sufficient for accumulation in the nucleus. When the `core' molecule was injected directly into the oocyte nucleus it remained there, indicating that the 'tail' region confers selective entry rather than selective retention. In the case of histone H1 a proteolytic fragment encompassing the carboxyterminal domain can accumulate in the nucleus. The amino acids lysine, proline and alanine comprise 75 of the 89 amino acids in this fragment. Since the remaining 14 amino acids are scattered throughout the fragment and not clustered any primary sequence specifying entry into the nucleus would seem necessarily to involve the amino acids lysine, proline and alanine. Positive charge alone cannot explain the accumulation of this gragment since poly L-lysine does not accumulate after microinjection into the cytoplasm. Fragments encompassing other domains of the molecule are so unstable in the oocyte that their ability to accumulate in the oocyte nucleus cannot be assayed. The gene for nucleoplasmin has been cloned and sequences have been found in the 'tail' region of nuceloplasmin that show homology to sequences identified in other nuclear proteins that appear to constitute a signal specifying nuclear localisation.