Functions as nuclear import receptor for serine-arginine rich (SR) proteins. Regulates nuclear import of SR proteins that are required for proper splicing of the two resistance (R) genes SNC1 and RPS4, a crucial step for their functions in plant immunity. Interacts with RS2Z33, RSZ21, RS31A, SR34 and RAN1. Semi-dwarf phenotype. Belongs to the importin beta family.