Tethering of eIF4G to adenoviral mRNAs by viral 100k protein drives ribosome shunting

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Abstract
Although most mRNAs initiate translation by 5′ ribosome scanning, some small fraction of mammalian and viral mRNAs utilize either of two alternate mechanisms, known as internal ribosome entry and ribosome shunting. Ribosome shunting is a poorly understood form of initiation in which 40S ribosome subunits are loaded onto mRNA through interactions with the m7GTP cap, but then bypass large segments of the mRNA as directed by cis-acting RNA shunting elements and trans-acting protein factors. Here, we describe the molecular mechanism by which ribosome shunting occurs with high efficiency on adenovirus late mRNAs. We show that the viral 100k protein possesses a selective binding element for the 5′ noncoding region (5′NCR) of viral late mRNAs (known as the tripartite leader), forms a complex with initiation factor eIF4G and poly(A)-binding protein (PABP), and strongly and selectively enhances the level of both factors and 40S ribosome subunits on viral mRNAs in polysomes. Mutational and biochemical studies demonstrate that the ability of 100k protein to bind both the tripartite leader and eIF4G are critical to promote a high level of ribosome shunting. A molecular mechanism for ribosome shunting is described by which enhanced binding of eIF4G and possibly PABP with 100k protein, and simultaneous interaction with the tripartite leader 5′NCR, drives 40S ribosome recruitment and initiation on mRNAs.