Targeting of most nuclear proteins to the cell nucleus is initiated by interaction between the protein's nuclear localization signal (NLS) and the importin, or karyopherin, receptor complex. An importin heterodimer recognizes the NLS protein in the cytoplasm via its alpha subunit and, via its beta subunit, docks the complex to a subset of peptide repeat-containing proteins known as nucleoporins.

Transportin-1 is  the beta subunit of the karyopherin receptor complex which interacts with nuclear localization signals to target nuclear proteins to the nucleus. The karyopherin receptor complex is a heterodimer of an alpha subunit which recognizes the nuclear localization signal and a beta subunit which docks the complex at nucleoporins.