We are using photobleaching techniques to measure dynamic properties of various proteins in the intact cell nucleus. We find that the vast majority of proteins move freely within the nucleoplasm by a passive, energy-independent mechanism.
Proteins rapidly associate and dissociate with specific nuclear subcompartments.
Our findings have revealed that proteins roam the nucleus to find appropriate binding partners and that their mobility is not restrained by structural components of the nucleus. We have now extended these methods to analyze the binding of various proteins to their targets in living cells. For example, we have for the first time measured the residence time of a chromatin binding protein, the linker histone H1, in unperturbed chromatin in a living cell. These methods now provide the basis for the molecular analysis of gene expression in the context of the intact cell nucleus.