Nuclear contents or cytoplasm from Xenopus oocytes labeled with (35-S)methionine or (3-H)proline (donor oocytes) were reinjected into unlabeled oocytes (recipient oocytes). The radioactivity injected as nuclear contents was found to enter and accumulate in the recipient oocyte nucleus. In contrast, the radioactivity injected as cytoplasm was found to enter but not to accumulate in the recipient oocyte nucleus. Sodium dodecyl sulfate (SDS) gel electrophoresis of the nucleus and cytoplasm of donor oocytes revealed the existence of three classes of labeled proteins in these oocytes: those proteins found predominantly in the nucleus (N proteins), those found predominantly in the cytoplasm (C proteins), and those found in both the nucleus and cytoplasm at similar concentrations (B proteins). SDS gel electrophoresis of the nucleus and cytoplasm of recipient oocytes showed that N proteins entered and accumulated in the nucleus but that B proteins partitioned about equally between the nucleus and cytoplasm. A similar analysis of oocytes injected with labeled cytoplasm showed that C proteins did not enter the nucleus but again B proteins partitioned about equally between the nucleus and cytoplasm.

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