Roles of mono-ubiquitinated Smad4 in the formation of Smad transcriptional complexes.
Ubiquitination is an increasingly common post-translation modification that controls both the expression and activity of numerous proteins in the eukaryotic cell. One frequent target of the ubiquitin (Ub) modification machinery is transcription factors. Although ubiquitination generally modulates their function by inducing proteasome-dependent degradation, past and recent studies indicate that ubiquitination also regulates nuclear-cytoplasmic trafficking of transcriptional regulators. Ubiquitination is known to modulate transcription factor localization by destroying sequestering proteins and by directly promoting nuclear import and export of modified substrates. This review discusses old and new paradigms relating Ub modification and the control of transcription factor shuttling in and out of the nucleus.