Histones constitute the protein components of nucleosomes. Despite their small sizes, histones do not diffuse through the nuclear pore complex. Instead, they are transported to the nucleus by importins, either alone or in complex with histone chaperones. Determining the molecular size of the importin-histone complexes is key to understanding the mechanism of histone transport and also the potential roles of importins as histone chaperones and in the assembly of nucleosomes. Here we report a simple and reproducible sedimentation-velocity based method to determine the molecular sizes of importin-histone complexes using analytical ultracentrifugation. The method does not use any reporter tags or interaction with column resin thereby analyzing the interactions of the native proteins.
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