The role of human and Drosophila NXF proteins in nuclear mRNA ...

Introduction 18
nucleoporins mediating the export-relevant interactions between the mRNA and the NPC
(Bachi et al., 2000; Bear et al., 1999; Katahira et al., 1999; Santos-Rosa et al., 1998;
Segref et al., 1997; Strasser et al., 2000; Strawn et al., 2001).
The functional and structural domain organization of TAP has been analyzed in
detail. TAP can be divided in two main functional parts: an N-terminal cargo-binding
domain (amino acids 1-372) and a C-terminal NPC-binding domain (amino acids 371-619;
Figure 4) (reviewed by Izaurralde, 2002).
The cargo-binding domain is composed of an N-terminal fragment which contains
a transportin-1-dependent NLS, an RNP-type RNA-binding domain (RBD, amino acids
119-198) and a leucine-rich repeat (LRR) domain (amino acids 203-362) (Bachi et al.,
2000; Liker et al., 2000; Truant et al., 1999). The crystal structure of a TAP fragment
comprising the RBD and the LRR domain has been solved recently (Ho et al., 2002; Liker
et al., 2000). The RBD and the LRR domain are both necessary and sufficient to mediate
the interaction with CTE-containing RNAs (amino acids 102-372). In contrast to this direct
binding, the specific recognition and stable association of TAP with cellular mRNAs is
most likely mediated by RNA-binding proteins. Factors suggested to serve as "adapters"
between TAP and the mRNAs include E1B-AP5, hGle2, members of the REF family and
other proteins present in the exon-exon junction complex (EJC) (Bachi et al., 2000;
Kataoka et al., 2001; Kataoka et al., 2000; Stutz et al., 2000; see also below). The
interactions with these adapters have been proposed to be mediated also by the cargobinding
domain of TAP. The deletion of individual domains in the N-terminal half of TAP
reduces the export activity of TAP. The removal of these domains has much weaker
negative effects when TAP is directly tethered to its RNA export substrates, demonstrating
the importance of these N-terminal domains for the efficient association with mRNAs
(Braun et al., 2001).
Figure 4: Structural and functional domains of human TAP
RBD: RNP-type RNA-binding domain (amino acids 119-198). LRR: leucine-rich repeat domain (amino
acids 203-362). NTF2: NTF2-like domain (amino acids 371-551). UBA: UBA-like domain (amino acids
563-619). The NTF2-like scaffold formed by the TAP:p15 interaction surface harbors one nucleoporinbinding
site, the UBA-like domain a second nucleoporin-binding site (modified from Herold et al.,
2001).