Molecular basis for the recognition of a nonclassical nuclear localization signal by importin β

Nuclear import of proteins containing a classical nuclear localization signal (NLS) involves NLS recognition by importin α, which associates with importin β via the IBB domain. Other proteins, including parathyroid hormone-related protein (PTHrP), are imported into the nucleus by direct interaction with importin β. We solved the crystal structure of a fragment of importin β-1 (1-485) bound to the nonclassical NLS of PTHrP. The structure reveals a second extended cargo binding site on importin β distinct from the IBB domain binding site. Using a permeabilized cell import assay we demonstrate that importin β (1-485) can import PTHrP-coupled cargo in a Ran-dependent manner. We propose that this region contains a prototypical nuclear import receptor domain, which could have evolved into the modern importin β superfamily.