Kyoung Mi Kim1,3
, Hana Cho1,3
, Kobong Choi2,3
, Jaedong Kim2
, Bong-Woo Kim1
, Young-Gyu Ko1
, Sung Key Jang2,5
and Yoon Ki Kim1,4
1School of Life Sciences and Biotechnology, Korea University, Seoul 136-701, Republic of Korea;
2Department of Life Science, Pohang University of Science and Technology, Hyoja Dong, Pohang, Kyungbuk 790-784, Republic of Korea
3 These authors contributed equally to this work.
During or right after mRNA export via the nuclear pore complex (NPC) in mammalian cells, mRNAs undergo translation mediated by nuclear cap-binding proteins 80 and 20 (CBP80/20). After CBP80/20-dependent translation, CBP80/20 is replaced by cytoplasmic cap-binding protein eIF4E, which directs steady-state translation. Nonsense-mediated mRNA decay (NMD), one of the best-characterized mRNA surveillance mechanisms, has been shown to occur on CBP80/20-bound mRNAs. However, despite the tight link between CBP80/20-dependent translation and NMD, the underlying molecular mechanism and cellular factors that mediate CBP80/20-dependent translation remain obscure. Here, we identify a new MIF4G domain-containing protein, CTIF (CBP80/20-dependent translation initiation factor). CTIF interacts directly with CBP80 and is part of the CBP80/20-dependent translation initiation complex. Depletion of endogenous CTIF from an in vitro translation system selectively blocks the translation of CBP80-bound mRNAs, while addition of purified CTIF restores it. Accordingly, down-regulation of endogenous CTIF abrogates NMD. Confocal microscopy shows that CTIF is localized to the perinuclear region. Our observations demonstrate the existence of CBP80/20-dependent translation and support the idea that CBP80/20-dependent translation is mechanistically different from steady-state translation through identification of a specific cellular protein, CTIF.
CTIF, nonsense-mediated mRNA decay, nuclear cap-binding protein CBP80/20, eukaryotic translation initiation factor 4G, steady-state translation
4 Corresponding authors.
5 Corresponding authors.
Article published online ahead of print. Article and publication date are is online at http://www.genesdev.org/cgi/doi/10.1101/gad.1823409.
Supplemental material is available at http://www.genesdev.org.
Received May 21, 2009. Accepted July 13, 2009.
Copyright © 2009 by Cold Spring Harbor Laboratory Press