Distribution of proteins between nucleus and cytoplasm of amoeba proteus

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Abstract
By transplanting nuclei between labeled and unlabeled cells, the major proteins of amoebas were localized, and 130 cellular proteins were identified by fluorography of 1-dimensional polyacrylamide electrophoretic gels; slightly less than half (designated NP, for nuclear proteins) are almost exclusively nuclear. Approximately 95% of the other proteins (designated CP, for cytoplasmic proteins) are roughly equally concentrated in the nucleus and cytoplasm, but, because the cytoplasm is 50 times larger than the nucleus, .apprx. 98% of each of the latter is in the cytoplasm. Of the CP, roughly 5% are not detectable in the nucleus. Assuming that these are restricted to the cytoplasm only because they are in structures too large to enter the nucleus, and labeled CP readily exit a nucleus introduced into unlabeled cytoplasm, it is concluded that the nuclear envelope does not limit the movement of any nonstructural cellular protein in either direction between the 2 compartments. Some NP are not found in the cytoplasm (although ostensibly synthesized there) presumably because of preferential binding within the nucleus. Almost 1/2 of the protein mass in nuclei in vivo is CP, and apparently only proteins of that group are lost from nuclei when cells are lysed. While an extracellular environment allows CP to exit isolated nuclei, the nuclear binding affinities for NP are retained. Further examination of NP distribution shows that many NP species are detectable in the cytoplasm (although at only .apprx. 1/300 the nuclear concentration), apparently because the nuclear affinity is relatively low. These proteins are electrophoretically distinguishable from the high-affinity NP not found in the cytoplasm. New experiments show that an earlier suggestion that the nuclear transplantation operation causes an artifactual release of NP to the cytoplasm is largely incorrect. Cytoplasmic contamination of nuclear preparations is not a factor in classifying proteins by these nuclear transplantation experiments. No mechanism has evolved to confine most CP to the cytoplasm (where they presumably function exclusively) because the cytoplasm''s large volume ensures that CP will be abundant there. Extending Bonner''s idea of quasi-functional nuclear binding sites for NP, a subset of NP probably have a low affinity for available intranuclear sites because their main function(s) occurs at other intranuclear sites to which they bind tightly only when particular metabolic conditions demand. The other NP (those completely absent from cytoplasm) presumably always are bound with high affinity at their primary functional sites.