Transportin SR2 (TRN-SR2) is a -type karyopherin responsible for the nuclear
import of specific cargoes, including serine/arginine-rich splicing factors. The
protein has been implicated in a variety of human diseases, including HIV
infection, primary biliary cirrhosis and limb-girdle muscular dystrophy 1F.
Towards understanding its molecular mechanism, a 2.9 A ˚ resolution crystal
structure of human TRN-SR2 complexed with the small GTPase Ran has been
determined. TRN-SR2 is composed of 20 -helical HEAT repeats forming a
solenoid-like fold. The first nine repeats form a ‘cradle’ for the binding of
RanGTP, revealing similarities but also differences with respect to the related
importin 13 complex.