Mobility of multi-subunit complexes in the nucleus: accessibility and dynamics of chromatin subcompartments.

The cell nucleus contains a number of mobile subnuclear organelles involved in RNA processing, transcriptional regulation and antiviral defence like Cajal and promyelocytic leukaemia (PML) bodies. It remains an open question how these bodies translocate to specific nuclear regions within the nucleus to exert their biological function. The mobility and localisation of macromolecules in the nucleus are closely related to the dynamic organisation and accessibility of chromatin. This relation has been studied with biologically inert fluorescent particles like dextrans, polystyrene nanospheres and inactive protein crystals formed by the Mx1-YFP fusion protein or other ectopically expressed proteins like vimentin. As reviewed here, properties of the chromatin environment can be identified from these experiments that determine the mobility of Cajal and PML bodies and other supramolecular complexes.