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J. Cell Biol.,
Volume 143, Number 2, October 19, 1998 309-318
Nuclear Import Factor
* Department of Genetics and Although importin Howard Hughes Medical Institute, Duke University Medical Center, Durham,
North Carolina 27710
(Imp ) has been
shown to act as the receptor for basic nuclear localization signals (NLSs) and to mediate their recruitment to
the importin 
nuclear import factor, little is known
about the functional domains present in Imp , with the exception that importin binding is known to map
close to the Imp NH2 terminus. Here, we demonstrate that sequences essential for binding to the CAS
nuclear export factor are located near the Imp COOH terminus and include a critical acidic motif.
Although point mutations introduced into this acidic
motif inactivated both CAS binding and Imp nuclear
export, a putative leucine-rich nuclear export signal
proved to be neither necessary nor sufficient for Imp nuclear export. Analysis of sequences within Imp that bind to the SV-40 T antigen NLS or to the similar LEF-1
NLS revealed that both NLSs interact with a subset of
the eight degenerate armadillo (Arm) repeats that form
the central part of Imp . However, these two NLS-binding sites showed only minimal overlap, thus suggesting that the degeneracy of the Arm repeat region
of Imp may serve to facilitate binding to similar but
nonidentical basic NLSs. Importantly, the SV-40 T NLS
proved able to specifically inhibit the interaction of Imp
with CAS in vitro, thus explaining why the SV-40
T NLS is unable to also function as a nuclear export signal.
;
nuclear export;
nuclear import;
nuclear localization signal
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