Regulation of transcription factor localization: fine-tuning of gene expression.

Import of 'nuclear' proteins into the nucleus, in particular, transcription factors, is not a constitutive process; instead it appears to be modulated in response to external stimuli, cell-cycle progression and developmental cues. Examples of such regulation involve direct phosphorylation of the transported protein, masking of the nuclear localization signal(s), cytoplasmic retention by binding to an anchoring protein, modulation of the import machinery itself and possible interplay between these different mechanisms. As such, nucleo-cytoplasmic traffic constitutes an important regulatory checkpoint in the control of gene expression.