Dbp5/DDX19 between Translational Readthrough and Nonsense Mediated Decay

Abstract

The DEAD-box protein Dbp5 (human DDX19) remodels RNA-protein complexes. Dbp5 functions in ribonucleoprotein export and translation termination. Termination occurs, when the ribosome has reached a stop codon through the Dbp5 mediated delivery of the eukaryotic termination factor eRF1. eRF1 contacts eRF3 upon dissociation of Dbp5, resulting in polypeptide chain release and subsequent ribosomal subunit splitting. Mutations in DBP5 lead to stop codon readthrough, because the eRF1 and eRF3 interaction is not controlled and occurs prematurely. This identifies Dbp5/DDX19 as a possible potent drug target for nonsense suppression therapy. Neurodegenerative diseases and cancer are caused in many cases by the loss of a gene product, because its mRNA contained a premature termination codon (PTC) and is thus eliminated through the nonsense mediated decay (NMD) pathway, which is described in the second half of this review. We discuss translation termination and NMD in the light of Dbp5/DDX19 and subsequently speculate on reducing Dbp5/DDX19 activity to allow readthrough of the PTC and production of a full-length protein to detract the RNA from NMD as a possible treatment for diseases.

Keywords: DDX19; Dbp5; NMD; Rat8; mRNA degradation; mRNA quality control; translation; translation termination.

Figure 1

Scheme of the domain structure of Dbp5. Indicated are the domains important for RNA- and ATP binding (red) and the domains important for the interaction partners and co-factors (grey).

Figure 2

Model for translation termination. The yeast DEAD-box RNA helicase Dbp5 delivers eRF1 to the ribosome located on a stop codon. Its function is to prevent an early contact of eRF1 with eRF3. As soon as eRF1 was placed properly, the ATP-hydrolysis of Dbp5 dissociates the helicase. Free eRF1 subsequently contacts eRF3 that waited in its GDP-bound form attached to Rli1. This contact triggers GTP-binding of eRF3 and subsequent hydrolysis causing polypeptide chain and tRNA release. At the same time, eIF3 delivers Hcr1, which eliminates eRF3 from the complex. Removal of eRF3 enables contact of eRF1 with Rli1-ATP and through ATP-hydrolysis the ribosomal subunits are split.

Figure 3

Lack of Dbp5 leads to translational readthrough. Mutations in Dbp5 prevent a shielded entry of eRF1 to the ribosome, resulting in an early contact of eRF1 and eRF3 and their subsequent dissociation. This increases the probability for the incorporation of a near-cognate tRNA and the continuation in translation.

Figure 4

Termination at a premature termination codon (PTC) and the subsequent initiation of the nonsense mediated decay (NMD) of the mRNA. Under normal conditions yeast Dbp5 promotes the stop codon recognition by eRF1 and eRF3 also at a PTC. However, when recognized as premature by the NMD machinery, Upf1 binds to the release factors and assembles the Upf1-2,3 complex. Consequently, further translation is inhibited, and downstream factors are recruited to promote rapid mRNA decay. The decapping factors Dcp1 and Dcp2 and Xrn1 act from 5′ to 3′ and the Ski complex and the exosome degrade the PTC-containing mRNA from 3′ into 5′ direction.

Figure 5

Affecting the availability of functional Dbp5 might generate full length proteins from PTC containing mRNAs. With a reduced function of Dbp5, the stop codon recognition of eRF1 and eRF3 is impaired. If the PTC is not recognized as a stop codon, a near cognate peptidyl-tRNA enters the ribosome, and translation elongation continues. A normal stop codon is less susceptible to translational readthrough than a PTC, presumably due to additional factors that promote translation termination, such as Pab1. Hence, there is a high probability that translation can terminate at the correct stop codon rather than at the PTC. This will most likely generate a full-length protein with only one amino acid exchange instead of a truncated polypeptide.