The binding site of karyopherin a for karyopherin β overlaps with a nuclear localization sequence

By using proteolysis, recombinant mutant proteins, or synthetic peptides and by testing these reagents in liquid phase binding or nuclear import assays, we have mapped binding regions of karyopherin α. We found that the C-terminal region of karyopherin α recognizes the nuclear localization sequence (NLS), whereas its N-terminal region binds karyopherin β. Surprisingly, karyopherin α also contains an NLS. Thus, karyopherin α belongs to a group of proteins that contain both a ligand (NLS) and a cognate receptor (NLS recognition site) in one molecule with a potential for autologous ligand-receptor interactions. The NLS of karyopherin α overlaps with the binding site of karyopherin a for karyopherin β. Hence, binding of karyopherin β to karyopherin α covers the NLS of karyopherin α. This prevents autologous ligand receptor interactions and explains the observed cooperative binding of karyopherin α to a heterologous NLS protein in the presence of karyopherin β.