The importin beta/importin 7 heterodimer is a functional nuclear import receptor for histone H1

Import of proteins into the nucleus proceeds through nuclear pore complexes and is largely mediated by nuclear transport receptors of the importin bet a family that use direct RanGTP-binding to regulate the interaction with th eir cargoes. We investigated nuclear import of the linker histone H1 and fo und that two receptors, importin beta (Imp beta) and importin 7 (Imp7, RanB P7), play a critical role in this process. Individually, the two import rec eptors bind H1 weakly, but binding is strong for the Imp beta/Imp7 heterodi mer. Consistent with this, import of H1 into nuclei of permeabilized mammal ian cells requires exogenous Imp beta together with Imp7, Import by the Imp 7/Imp beta heterodimer is strictly Ran dependent, the Ran-requiring step mo st likely being the disassembly of the cargo-receptor complex following tra nslocation into the nucleus. Disassembly is brought about by direct binding of RanGTP to Imp beta and Imp7, whereby the two Ran-binding sites act syne rgistically, However, whereas an Imp beta/RanGTP interaction appears essent ial for H1 import, Ran-binding to Imp7 is dispensable. Thus, Imp7 can funct ion in two modes. Its Ran-binding site is essential when operating as an au tonomous import receptor, i.e. independently of Imp beta. Within the Imp be ta/Imp7 heterodimer, however, Imp7 plays a more passive role than Imp beta and resembles an import adapter.