U. Kutay et al., DOMINANT-NEGATIVE MUTANTS OF IMPORTIN-BETA BLOCK MULTIPLE PATHWAYS OFIMPORT AND EXPORT THROUGH THE NUCLEAR-PORE COMPLEX, EMBO journal, 16(6), 1997, pp. 1153-1163
Nuclear protein import proceeds through the nuclear pore complex (NPC)
, Importin-beta mediates translocation via direct interaction with NPC
components and carries importin-alpha with the NLS substrate from the
cytoplasm into the nucleus, The import reaction is terminated by the
direct binding of nuclear RanGTP to importin-beta which dissociates th
e importin heterodimer, Here, we analyse the sites of interaction on i
mportin-beta for its multiple partners, Ran and importin-alpha respect
ively require residues 1-364 and 331-876 of importin-beta for binding,
Thus, RanGTP-mediated release of importin-alpha from importin-beta is
likely to be an active displacement rather than due to simple competi
tion between Ran and importin-alpha for a common binding site, Importi
n-beta has at least two non-overlapping sites of interaction with the
NPC, which could potentially be used sequentially during translocation
, Our data also suggest that termination of import involves a transien
t release of importin-beta from the NPC, Importin-beta fragments which
bind to the NPC, but not to Ran, resist this release mechanism, As wo
uld be predicted from this, these importin-beta mutants are very effic
ient inhibitors of NLS-dependent protein import, Surprisingly, however
, they also inhibit M9 signal-mediated nuclear import as well as nucle
ar export of mRNA, U snRNA, and the NES-containing Rev protein, This s
uggests that mediators of these various transport events share binding
sites on the NPC and/or that mechanisms exist to coordinate transloca
tion through the NPC via different nucleocytoplasmic transport pathway
s.