Ubiquitylation, a protein modification most familiar for signaling proteasomal degradation, is used to regulate many other cellular processes as well. Dantuma et al. expressed a ubiquitin-green fluorescent protein fusion protein (GFP-Ub) in Mel JuSo human melanoma cells to investigate how the use of ubiquitin in one cell process affects its availability for others. Both Western analysis and fluorescence loss in photobleaching analysis of GFP-Ub exchange between the nucleus and cytoplasm suggested that only a small fraction of cellular ubiquitin existed as free monomers. Quantitative fluorescence recovery after photobleaching analysis indicated that about 70% of nuclear GFP-Ub was immobile (and thus likely bound to histones). Exposure of cells to the proteasome inhibitor MG132 (which causes proteotoxic stress and loss of ubiquitin from histones) elicited a decrease in the size of the immobilized nuclear pool of GFP-Ub, redistribution of GFP-Ub from the nucleus to the cytoplasm, and a decrease in GFP-Ub diffusion rate (to a rate consistent with association with proteasomes). Western analysis confirmed a loss in ubiquitylated histone H2A with kinetics similar to the loss of GFP-Ub-histone. However, analysis of the disappearance of a Ub fusion protein containing photoactivatable GFP from a region of the nucleus indicated that MG132 did not affect the rate of histone deubiquitylation. Further, microinjection of an antibody to GFP into the cytosol also led to the depletion of nuclear GFP-Ub and the accumulation of cytosolic GFP-Ub. Thus, the authors propose that competition among ubiquitin-dependent processes for a limited pool of free ubiquitin may provide a mechanism linking proteotoxic stress to broad changes in gene expression.
N. P. Dantuma, T. A. M. Groothuis, F. A. Salomons, J. Neefjes, A dynamic ubiquitin equilibrium couples proteasomal activity to chromatin remodeling. J. Cell Biol. 173, 19-26 (2006). [Abstract] [Full Text]