Characterization of the nuclear import of human CHD4-NuRD complex.

Chromatin remodeling enzymes form large multiprotein complexes that play central roles in regulating access to the genome. Here, we characterize the nuclear import of the human CHD4 protein. We show that CHD4 enters the nucleus by means of several importin alpha proteins (1, 5, 6, and 7), and rather independently of importin beta 1. Importin alpha 1 directly interacts with a monopartite "KRKR"-motif in the N-terminus of CHD4 (aa 304-307). However, alanine mutagenesis of this motif only leads to approx. 50% reduction in nuclear localization of CHD4, implying additional import mechanisms. Interestingly, we can show that CHD4 is already associated with NuRD core subunits such as MTA2, HDAC1, or RbAp46 in the cytoplasm, suggesting an assembly of the NuRD core complex before nuclear import. We propose that, in addition to the importin-alpha-dependent nuclear localization signal, CHD4 is dragged into the nucleus by a "piggyback mechanism" using the import signals of the associated NuRD subunits.