NUCLEAR EVENTS Trapped in the Nucleolus by GTP

Nucleostemin, a protein found in the nucleoli of highly proliferative cells (such as stem cells and some cancer cell lines), may help regulate cell proliferation. Tsai and McKay, who previously identified nucleostemin, have now investigated the mechanisms whereby it is targeted to the nucleolus. The authors used fluorescence recovery after photobleaching (FRAP) and inverse FRAP (iFRAP) to show that nucleostemin tagged with green fluorescent protein shuttled rapidly and bidirectionally between the nucleolus and the nucleoplasm of cultured CHO and U2 OS cells. Mutation of a GTP-binding motif (G1) that blocked the ability of nucleostemin to bind GTP also blocked its nucleolar localization, as did deletion of an N-terminal basic (B) domain. FRAP, together with further mutational analysis, indicated that GTP binding relieved the inhibitory action of a domain between G1 and the C terminal on nucleolar localization of the B domain and was required for long-term retention of nucleostemin in the nucleus. Further, decreasing the concentration of intracellular GTP shortened the residence time of nucleostemin in the nucleolus and reduced its nucleolar localization. Thus the authors propose that nucleolar residence of nucleostemin involves a cycle driven by its GTP-binding status and suggest that this provides a mechanism to rapidly and reversibly regulate the transition of stem cells between resting and proliferating states. Misteli provides context on the research in a Comment, emphasizing the importance of investigating the relation between cell signaling and the regulation of subnuclear localization of proteins.

R. Y. L. Tsai, R. D. G. McKay, A multistep, GTP-driven mechanism controlling the dynamic cycling of nucleostemin. J. Cell Biol. 168, 179-184 (2005). [Abstract] [Full Text]

T. Misteli, Going in GTP cycles in the nucleolus. J. Cell Biol. 168, 177-178 (2005). [Abstract] [Full Text]

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PERSPECTIVES
Sensing Cellular Stress: Another New Function for the Nucleolus?

Mark O. J. Olson (16 March 2004)
Sci. STKE 2004 (224), pe10. [DOI: 10.1126/stke.2242004pe10]
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