The importin β / importin 7 heterodimer is a functional nuclear import receptor for histone H 1

Import of proteins into the nucleus proceeds through nuclear pore complexes and is largely mediated by nuclear transport receptors of the importin β family that use direct RanGTP-binding to regulate the interaction with their cargoes. We investigated nuclear import of the linker histone H1 and found that two receptors, importin β (Impβ) and importin 7 (Imp7, RanBP7), play a critical role in this process. Individually, the two import receptors bind H1 weakly, but binding is strong for the Impβ/Imp7 heterodimer. Consistent with this, import of H1 into nuclei of permeabilized mammalian cells requires exogenous Impβ together with Imp7. Import by the Imp7/Impβ heterodimer is strictly Ran dependent, the Ran-requiring step most likely being the disassembly of the cargo– receptor complex following translocation into the nucleus. Disassembly is brought about by direct binding of RanGTP to Impβ and Imp7, whereby the two Ranbinding sites act synergistically. However, whereas an Impβ/RanGTP interaction appears essential for H1 import, Ran-binding to Imp7 is dispensable. Thus, Imp7 can function in two modes. Its Ran-binding site is essential when operating as an autonomous import receptor, i.e. independently of Impβ. Within the Impβ/ Imp7 heterodimer, however, Imp7 plays a more passive role than Impβ and resembles an import adapter.