Mechanism for the selection of nuclear polypeptides in Xenopus oocytes

The function of the nuclear envelope in regulating the cellular distribution of proteins was studied by experimentally altering nuclear permeability and determing the effect of the procedure on the incorporation of exogenous and endogenous polypeptides into the nucleoplasm. Using fine glass needles, nuclear envelopes were disrupted by puncturing oocytes in that region of the animal pole occupied by the germinal vesicle. This resulted in a highly significant increase in the nuclear uptake of cytoplasmically injected [125I]-bovine serum albumin ([125I]BSA), deomonstrating that the envelopes had lost their capacity to act as effective barriers to the diffusion of macromolecules. Endogenous proteins were labeled by incubating oocytes in L-[3H]lecuine. After appropriate intervals, nuclei were isolated from punctured and control cells and analyzed for tritiated polypeptides. Both total precipitable counts and the proportion of label in different size classes of polypeptides were compared. The results showed that puncturing the oocytes had no apparent quantitative or qualitative effects on the uptake of endogenous polypeptides by the nuclei. It can be concluded that the accumulation of specific nuclear proteins is not controlled by the envelope but rather by selective binding within the nucleoplasm.