HnRNP core proteins: synthesis, turnover and intracellular distribution.

Our present data indicate that the Mr 34-40,000 polypeptides which are involved in the binding of a large fraction of hnRNA sequences, including mRNA, are for the most part metabolically stable species in mouse ascites tumor cells. An exception to this generalization is the smallest of 30S RNP core polypeptides, the Mr 34,000 protein, which has a relatively high turnover rate. The relationship of the various synthesis and degradation rates to the physiological state of mammalian cells remains to be determined, as does the pathway of assembly and disassembly of RNP substructures during re-utilization of the proteins and during their turnover. Immunofluorescent studies, which have confirmed the expected nucleoplasmic or euchromatic localization of the RNP core proteins, have also indicated that these species are stable during mitosis, at which time they are dispersed through the cell away from the condensed chromosomes. The proteins appear to relocate in the nucleus as soon as the nuclear envelope is reformed.