Kap120 functions as a nuclear import receptor for ribosome assembly factor Rpf1 in yeast.

The nucleocytoplasmic exchange of macromolecules is mediated by receptors specialized in passage through the nuclear pore complex. The majority of these receptors belong to the importin beta protein family, which has 14 members in Saccharomyces cerevisiae. Nine importins carry various cargos from the cytoplasm into the nucleus, whereas four exportins mediate nuclear export. Kap120 is the only receptor whose transport cargo has not been found previously. Here, we characterize Kap120 as an importin for the ribosome maturation factor Rpf1, which was identified in a two-hybrid screen. Kap120 binds directly to Rpf1 in vitro and is released by Ran-GTP. At least three parallel import pathways exist for Rpf1, since nuclear import is defective in strains with the importins Kap120, Kap114, and Nmd5 deleted. Both kap120 and rpf1 mutants accumulate large ribosomal subunits in the nucleus. The nuclear accumulation of 60S ribosomal subunits in kap120 mutants is abolished upon RPF1 overexpression, indicating that Kap120 does not function in the actual ribosomal export step but rather in import of ribosome maturation factors.