Kap121p (also known as Pse1p) is an essential karyopherin that mediates nuclear
import of a plethora of cargoes including cell cycle regulators, transcription
factors, and ribosomal proteins in Saccharomyces cerevisiae. It has been
proposed that the spindle assembly checkpoint signaling triggers molecular
rearrangements of nuclear pore complexes and thereby arrests Kap121p-mediated
nuclear import at metaphase, while leaving import mediated by other karyopherins
unaffected. The Kap121p-specific import inhibition is required for normal
progression through mitosis. To understand the structural basis for
Kap121p-mediated nuclear import and its unique regulatory mechanism during
mitosis, we determined crystal structures of Kap121p in isolation and also in
complex with either its import cargoes or nucleoporin Nup53p or RanGTP. Kap121p
has a superhelical structure composed of 24 HEAT repeats. The structures of
Kap121p-cargo complexes define a non-conventional nuclear localization signal
(NLS) that has a consensus sequence of KV/IxKx1-2K/H/R. The structure of
Kap121p-Nup53p complex shows that cargo and Nup53p compete for the same
high-affinity binding site, explaining how Nup53p binding forces cargo release
when the Kap121p-binding site of Nup53p is exposed during mitosis. Comparison of
the NLS and RanGTP complexes reveals that RanGTP binding not only occludes the
cargo-binding site but also forces Kap121p into a conformation that is
incompatible with NLS recognition.
