Mammalian heterogeneous nuclear RNP (hnRNP) subcomplexes are shown to be comprised of 14-17 basic A and B core group polypeptides (chrp) when subjected to two-dimensional immunoblot analysis. These proteins are normally confined to the nucleus but are distributed throughout the cell during mitosis. However, not all of the 17 protein spots are observed for all stages of the cell cycle. HeLa cell populations have been synchronized and the basic hnRNP core protein complement examined during S, G2, mitosis, and G1. During cell division several distinct chrp polypeptide species at 35 and 37 kD appear, while another of 37 kD and a chrp of 38 kD are diminished. These altered chrp complements are not due to any effects induced by thymidine treatment but appear to be physiological changes in the chrp polypeptide modification state. The new charge isomers found during mitosis are not the result of selective phosphorylation of the chrp polypeptides. However the nature of the modifications has yet to be determined. The mitosis-specific modified forms of the chrp polypeptides are found in the cytoplasmic fraction derived from mitotic cell populations. When this fraction is centrifuged upon sucrose density gradients the modified chrp polypeptides sediment from 30-200S in a distribution similar to that of hnRNP complexes isolated from the nuclei of randomly dividing cell populations. RNase digestion experiments indicate that the general substructure of the RNA/protein complexes in mitotic cell cytoplasm is similar to that of nuclear hnRNP isolated from unsynchronized cells or tissue.

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