The decrease in abundance of a subset of highly conserved basic nuclear proteins is established to correlate with the loss of proliferative potential in association with the process of terminal differentiation in murine mesenchymal stem cells and human keratinocytes. These proteins, designated P2Ps for proliferation potential proteins, have apparent molecular masses of 30-40 kD, are associated with the 30-40S substructures of nuclear hnRNP complexes, and are recognized by antibodies made against core proteins of hnRNP particles. They also share an epitope in common with heat shock protein-90 (hsp90) and are recognized by two mAbs against hsp90. Two-dimensional electrophoretic Western blots furthermore show that P2Ps make up a subset of hnRNP proteins. Cells that possess these proteins express the potential to proliferate whether or not they are traversing the cell cycle. These include rapidly growing cells, reversibly growth-arrested cells, and nonterminally differentiated cells. In contrast, cells that have irreversibly lost their proliferative potential, such as terminally differentiated cells, show a marked reduction in the abundance of P2Ps as determined by immunodetection on Western blots. A correlation, therefore, exists between the presence of this subset of nuclear proteins and the proliferative potential in two cell types. These results raise the possibility that as a subset of hnRNP proteins, P2Ps may mediate posttranscriptional control of the processing of specific RNAs required for cell proliferation.

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