P2P‐R protein localizes to the nucleolus of interphase cells and the periphery of chromosomes in mitotic cells which show maximum P2P‐R immunoreactivity

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Abstract
P2P‐R is a nuclear protein that can bind both p53 and Rb1. Its functions include roles in the control of RNA metabolism, apoptosis, and p53‐dependent transcription. The expression of P2P‐R also is repressed in G1 arrested terminally differentiated cells. The current studies therefore evaluated if P2P‐R undergoes cell cycle‐associated changes in its abundance and/or localization. Western blots show that relative to G0 quiescent cells, P2P‐R protein levels are higher in populations of G2/M cells prepared by the physiological parasynchronization technique of serum deprivation followed by serum stimulation. More striking is the > 10‐fold enrichment of P2P‐R protein in specimens of highly purified mitotic cells prepared by the mitotic shake‐select technique, or by synchrony with the mitotic spindle disruption agents nocodazole or vinblastine. These changes in P2P‐R protein occur without a concomitant change in P2P‐R mRNA expression suggesting that P2P‐R immunoreactivity increases during mitosis. Confocal microscopy next established the localization of P2P‐R to nucleoli in interphase cells and at the periphery of chromosomes in mitotic cells that lack nucleoli. The high levels of P2P‐R localized to the periphery of chromosomes in mitotic cells suggest that P2P‐R shares characteristics with other nucleolar proteins that associate with the periphery of chromosomes during mitosis. These include: nucleolin, B23, Ki67, and fibrillarin. J. Cell. Physiol. 191: 145–154, 2002.