Schematic representation of two phosphorylation-activated transport signals that confer nuclear import independent of importin β1. (A) ERK1/2 (cyan) is sequestered in the cytoplasm by anchoring proteins (shown in grey). Upon phosphorylation at its TEY site, ERK1/2 undergoes a conformational change, causing it to dissociate from the anchoring proteins. This allows MEK access to the NTS which it phosphorylates. Upon NTS phosphorylation, ERK1/2 is recognized and imported into the nucleus by importin β7 (yellow). (B) The SR protein ASF/SF2 (cyan) is phosphorylated at Arg-Ser (RS) dipeptide repeats that function as an NLS for the import receptor importin β2 (also known as transportin), shown in purple.