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which we identified by yeast two-hybrid screening for interactions with components
of cellular nucleocytoplasmic transport pathways. Further experiments characterized
these interactions as nonconventional but functionally relevant. This is the UL69
protein of HCMV, which interacts with the cellular mRNA export factor UAP56
(Lischka et al. 2006b), and the essential viral regulatory factor pUL84, which interacts
via a nonconventional nuclear localization domain with importin-α proteins (Lischka
et al. 2003b). The relevance of these interactions for viral replication and the poten-
tial implications for antiviral therapeutic approaches will be discussed.
Nuclear Import and Export Pathways
Eukaryotic cells use several nuclear transport pathways, each of which transports a
specific range of macromolecules (proteins or various RNA species) either into or
out of the nucleus. Three key steps are common to all nuclear trafficking pathways:
(1) generation of a complex consisting of cargo and carrier, (2) translocation of this
complex through the NPC, and (3) release of the cargo in the target compartment.
Most pathways use a homologous family of carrier molecules collectively called
β-karyopherins with import carriers called importins and export carriers called export-
ins (Radu et al. 1995; Gorlich et al. 1994; Stade et al. 1997). The export of mRNA is
unique, since it primarily uses the TAP/p15 heterodimer as the main mRNA export
receptor, which is not related to β-karyopherins (Gruter et al. 1998).
Protein Import and Export
The best characterized nucleocytoplasmic transport pathway is the classical nuclear
import of proteins (Fig. 1). In a first step, importin-α proteins, which act as adaptor
molecules, interact with protein cargos via targeting sequences called nuclear
localization signals (NLSs). Classical NLSs consist of either one (monopartite) or
two (bipartite) short stretches of basic amino acids (for a review see Lange et al.
2007). Molecular recognition of NLSs is crucial for the formation of the import
complex and is mediated via specific sites on importin-α that are located within the
armadillo repeat region of the protein (Conti et al. 1998; Fontes et al. 2000).
Importin-α links the cargo to the β-karyopherin importin-β, which mediates interac-
tion of the trimeric complex with the NPC and translocates the cargo into the
nucleus. Within the nucleus, importin-β then binds to RanGTP, inducing a confor-
mational change that results in the release of the cargo. Importin-β complexed with
RanGTP is recycled to the cytoplasm, whereas importin-α is exported complexed
with the β-karyopherin CAS and RanGTP. Finally, cytoplasmic RanGAP (Ran
GTPase-activating protein) stimulates the Ran GTPase, which generates RanGDP
and thus releases the importins within the cytoplasm for additional import cycles
(reviewed in Stewart 2007).
 
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