RESUMEN
Import-Karyopherin or Importin proteins bind nuclear localization signals (NLSs) to mediate the import of proteins into the cell nucleus. Karyopherin ß2 or Kapß2, also known as Transportin, is a member of this transporter family responsible for the import of numerous RNA binding proteins. Kapß2 recognizes a targeting signal termed the PY-NLS that lies within its cargos to target them through the nuclear pore complex. The recognition of PY-NLS by Kapß2 is conserved throughout eukaryotes. Kap104, the Kapß2 homolog in Saccharomyces cerevisiae, recognizes PY-NLSs in cargos Nab2, Hrp1, and Tfg2. We have determined the crystal structure of Kapß2 bound to the PY-NLS of the mRNA processing protein Nab2 at 3.05-Å resolution. A seven-residue segment of the PY-NLS of Nab2 is observed to bind Kapß2 in an extended conformation and occupies the same PY-NLS binding site observed in other Kapß2·PY-NLS structures.