Retinoblastoma tumor suppressor: analyses of dynamic behavior in living cells reveal multiple modes of regulation.

Abstract	The retinoblastoma tumor suppressor, RB, assembles multiprotein complexes to mediate cell cycle inhibition. Although many RB binding partners have been suggested to underlie these functions, the validity of these interactions on the behavior of RB complexes in living cells has not been investigated. Here, we studied the dynamic behavior of RB by using green fluorescent protein-RB fusion proteins. Although these proteins were universally nuclear, phosphorylation or oncoprotein binding mediated their active exclusion from the nucleolus. In vivo imaging approaches revealed that RB exists in dynamic equilibrium between a highly mobile and a slower diffusing species, and genetic lesions associated with tumorigenesis increased the fraction of RB in a highly mobile state. The RB complexes dictating cell cycle arrest were surprisingly dynamic and harbored a relatively short residence time on chromatin. In contrast, this rapid exchange was attenuated in cells that are hypersensitive to RB, suggesting that responsiveness may inversely correlate with mobility. The stability of RB dynamics within the cell was additionally modified by the presence and function of critical corepressors. Last, the RB-assembled complexes present in living cells were primarily associated with E2F binding sites in chromatin. In contrast to RB, E2F1 consistently maintained a stable association with E2F sites regardless of cell type. Together, these results elucidate the kinetic framework of RB tumor suppressor action in transcriptional repression and cell cycle regulation.
Authors	Steven P Angus, David A Solomon, Lioba Kuschel, Robert F Hennigan, Erik S Knudsen
Journal	Molecular and cellular biology (Mol Cell Biol) Vol. 23 Issue 22 Pg. 8172-88 (Nov 2003) ISSN: 0270-7306 [Print] United States
PMID	14585976 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't, Research Support, U.S. Gov't, P.H.S.)
Chemical References	Cell Cycle Proteins Chromatin Cyclin E DNA-Binding Proteins E2F Transcription Factors E2F1 Transcription Factor E2F1 protein, human E2f1 protein, rat Luminescent Proteins Macromolecular Substances Recombinant Fusion Proteins Repressor Proteins Retinoblastoma Protein Transcription Factors Green Fluorescent Proteins Cyclin-Dependent Kinases
Topics	Animals Cell Cycle Cell Cycle Proteins Cell Line Cell Movement Cell Nucleolus (metabolism) Cell Nucleus (metabolism) Chromatin (metabolism) Cyclin E (metabolism) Cyclin-Dependent Kinases (metabolism) DNA-Binding Proteins E2F Transcription Factors E2F1 Transcription Factor Genes, Retinoblastoma Green Fluorescent Proteins Humans Luminescent Proteins (genetics, metabolism) Macromolecular Substances Models, Biological Phosphorylation Rats Recombinant Fusion Proteins (chemistry, genetics, metabolism) Repressor Proteins (metabolism) Retinoblastoma Protein (chemistry, genetics, metabolism) Transcription Factors (metabolism) Transcription, Genetic Transfection

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