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* Cold Spring Harbor Laboratory, Cold Spring Harbor, New York 11724; and The perinucleolar compartment (PNC) is a
unique nuclear structure preferentially localized at the
periphery of the nucleolus. Several small RNAs transcribed by RNA polymerase III (e.g., the Y RNAs,
MRP RNA, and RNase P H1 RNA) and the polypyrimidine tract binding protein (PTB; hnRNP I) have thus
far been identified in the PNC (Ghetti, A., S. PinolRoma, W.M. Michael, C. Morandi, and G. Dreyfuss.
1992. Nucleic Acids Res. 20:3671-3678; Matera, A.G., M.R. Frey, K. Margelot, and S.L. Wolin. 1995. J. Cell
Biol. 129:1181-1193; Lee, B., A.G. Matera, D.C. Ward,
and J. Craft. 1996. Proc. Natl. Acad. Sci. USA. 93:
11471-11476). In this report, we have further characterized this structure in both fixed and living cells. Detection of the PNC in a large number of human cancer and
normal cells showed that PNCs are much more prevalent in cancer cells. Analysis through the cell cycle using immunolabeling with a monoclonal antibody, SH54,
specifically recognizing PTB, demonstrated that the
PNC dissociates at the beginning of mitosis and reforms
at late telophase in the daughter nuclei. To visualize the
PNC in living cells, a fusion protein between PTB and
green fluorescent protein (GFP) was generated. Time
lapse studies revealed that the size and shape of the
PNC is dynamic over time. In addition, electron microscopic examination in optimally fixed cells revealed that the PNC is composed of multiple strands, each
measuring ~80-180 nm diam. Some of the strands are
in direct contact with the surface of the nucleolus. Furthermore, analysis of the sequence requirement for targeting PTB to the PNC using a series of deletion mutants of the GFP-PTB fusion protein showed that at
least three RRMs at either the COOH or NH2 terminus
are required for the fusion protein to be targeted to the
PNC. This finding suggests that RNA binding may be
necessary for PTB to be localized in the PNC.
The National Center for Microscopy and Imaging
Research at San Diego, University of California, La Jolla, California 92093
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