The control of gene expression requires the proper regulation of translation and mRNA degradation. Translation and mRNA degradation are inversely related due to the dual nature of the cap and poly(A) tail structures in promoting translation initiation, and as targets of the mRNA degradation machinery. Translation initiation is promoted by the binding of the eIF4F complex to the cap structure thereby allowing the eIF4G subunit to serve as a scaffold to recruit the 43S pre-initiation multi-factor complex (MFC), which includes the small ribosomal subunit, the initiator tRNA, and additional initiation factors (reviewed in Sonenberg and Hinnebusch, 2009
). Translation initiation is also enhanced by the poly(A) tail, in part through interactions of the poly(A) binding protein (Pab1) with eIF4G (Tarun and Sachs, 1996
). Conversely, mRNA degradation is generally initiated by deadenylation followed by decapping and 5′ to 3′ degradation (Parker and Song, 2004
; Garneau et al., 2007
). Decapping is enhanced by inhibition of translation initiation and loss of the eIF4F cap-binding complex, which presumably increases the availability of the cap to the decapping enzyme (Schwartz and Parker, 1999
The transition of mRNAs between translation and decapping by the Dcp1/Dcp2 decapping enzyme is promoted by a conserved set of decapping activator proteins including Dhh1/Rck, Pat1, Scd6/RAP55, the Lsm1-7 complex, and in metazoans Ge-1 (reviewed in Franks and Lykke-Andersen, 2008
). These proteins physically interact with each other, and can both directly inhibit translation, and/or stimulate the decapping enzyme (Coller and Parker, 2005
; Nissan et al., 2010
; Tritschler et al., 2009
). In addition, in metazoans, the NudT16 protein functions as a second decapping enzyme but how its activity is regulated remains to be determined (Song et al., 2010). Some of these translation repressor/decapping activator proteins also function in the storage and/or transport of mRNAs and can accumulate with translationally repressed mRNAs in dynamic cytoplasmic RNP granules referred to as P-bodies (reviewed in Eulalio et al., 2007
; Anderson and Kedersha, 2009
; Parker and Sheth, 2007
). An unresolved issue is how translation repressor/decapping activator proteins repress translation and can both target mRNAs for decapping or mRNA storage.
A highly conserved component of the translation repression/mRNA decay machinery is the Scd6 protein family, whose orthologs in Drosophila (TraI), nematodes (CAR-1), humans (RAP55), and plants (DCP5) are involved in translation repression and mRNA storage, and accumulate in cytoplasmic mRNP granules related to P-bodies (Xu and Chua, 2009
; Boag et al., 2005
; Wilhelm et al., 2005
; Barbee et al., 2006
; Tanaka et al., 2006
; Tritschler et al., 2007
). Moreover, the planaria ortholog of Scd6 is required for maintenance of stem cell potential, possibly through the storage of mRNAs (Wang et al., 2010
). In yeast and Arabidopsis, Scd6 and its plant ortholog (DCP5) promote mRNA decapping (Decourty et al., 2008
; Xu and Chua, 2009
). In contrast, in plasmodium the Scd6 ortholog is required for the stable storage of maternal mRNAs (Mair et al., 2010).
The Scd6 family members have a conserved domain organization with an N terminal Lsm domain, a central FDF motif, which interacts with a conserved RNA helicase referred to as Dhh1 in yeast (Tritschler et al., 2009
), and a C terminal RGG domain. Both yeast and plant Scd6 family members can bind the Dcp2 decapping enzyme but are not sufficient to directly activate decapping activity (Nissan et al., 2010
; Xu and Chua, 2009
). In contrast, Scd6 family members from yeast, Arabidopsis and Xenopus can directly repress translation in vitro (Nissan et al., 2010
, Xu and Chua, 2009
; Tanaka et al., 2006
), and at least for the yeast protein, this repression requires the C-terminal RGG domain and blocks translation initiation prior to formation of the 48S pre-initiation complex (Nissan et al., 2010
). An unresolved issue is the mechanism by which Scd6 represses translation and leads to stable mRNA storage.
In this work, we show that Scd6 directly inhibits translation by binding the eIF4G subunit of eIF4F through its RGG domain, thereby forming an mRNP repressed for translation initiation. Such a complex also provides a possible mechanism by which Scd6 (and possibly its orthologs) can stably store translationally repressed mRNAs. In addition, we demonstrate that two other RGG domain containing proteins, Npl3 and Sbp1, also directly bind eIF4G and repress translation via their RGG-motifs. These observations identify the mechanism of Scd6 function and indicate that eIF4G also plays an important scaffolding protein for the recruitment of translation repressors with RGG-motifs.