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Fig. 4
Models of ICP27- and pUL69-mediated viral mRNA export. ICP27 of HSVI binds
directly to viral intronless transcripts. Via its interaction with the adaptor protein REF, the RNA
is recruited to the cellular TAP-p15 mRNA export receptor, thus forming an export-competent
mRNP. The UL69 protein of HCMV interacts both with the cellular transcription elongation factor
hSPT6 and the mRNA export factor UAP56. Thus, pUL69 may facilitate mRNA export by opti-
mizing the co-transcriptional loading of RNA export factors to nascent mRNA
named hIws1, the depletion of which induces nuclear retention of poly(A) RNA.
This observation suggests that hSPT6 integrates transcription elongation with
downstream mRNA export (Yoh et al. 2007). However, with respect to the docu-
mented interaction of ICP27 with Aly/REF, it was even more interesting to identify
the putative DExH/D box RNA helicase URH49 as a binding protein of pUL69 by
yeast two-hybrid screening. URH49 is highly related to the mRNA export protein
UAP56, which is located upstream of Aly/REF in the cellular mRNA export path-
way (see Fig. 2) (Pryor et al. 2004). In both yeast two-hybrid and co-immunopre-
cipitation experiments, we demonstrated that pUL69 interacts not only with
URH49, but also with UAP56 (Lischka et al. 2006b). Although details on the bio-
logical function of URH49 are unknown so far, initial data support the assumption
that both DExH/D box proteins exert largely overlapping functions in the process-
ing and export of mammalian mRNAs (Kapadia et al. 2006; Pryor et al. 2004). In
this context, it should be noted that UAP56, alternatively termed BAT1, has been
