Functional and structural studies of importins in complex with polycationic substrates

Nuclear transport receptors of the Impβ-superfamily are responsible for import and export of proteins into and from the nucleus of eukaryotic cells. Importins have an additional function: they prevent the aggregation of positively charged proteins, such as histones and ribosomal proteins. The exact nature how importins can shield their cargo from undesired interactions is so far unknown. During this study, the interactions between several importins and their cargoes were further studied by characterization of their stability and finally screening for crystallization conditions. The known complexes of Imp9/rpS7, Imp4/rpS3a & Impβ/7/H1.0 were found to be resistant against high concentrations of salt, further proving that several ionic interactions are involved. The Impβ/7 dimer was found to be unstable in low salt concentrations, indicating that the interaction between these two importins is hydrophobic. In addition, nanobodies binding Impβ/7 have been generated as new tools to study the dimer. These nanobodies were tested for their usability in different roles. Nanobodies binding Imp7 inhibited the transport of H1.0. Additionally, two nanobodies show potential to be used as a synthetic cargo of Impβ/7. Imp7-binding nanobodies together with the cargo H1.0 were used to stabilize Impβ/7 and found to be helpful as promising conditions have been identified. These are promising results, which highlight the potential of nanobodies in research of nuclear transport receptors.