Alternatively spliced TCR mRNA induced by disruption of reading frame

Nuclear translation: what is the evidence?

Recently, several reports have been published in support of the idea that protein synthesis occurs in both the nucleus and the cytoplasm. This proposal has generated a great deal of excitement because, if true, it would mean that our thinking about the compartmentalization of cell functions would have to be re-evaluated. The significance and broad implications of this phenomenon require that the experimental evidence used to support it be carefully evaluated. Here, we critique the published evidence in support of, or in opposition to, the question of whether translation occurs in the nucleus. Arguments in support of nuclear translation focus on three issues: (1) the presence of translation factors and ribosomal components in the nucleus, and their recruitment to sites of transcription; (2) amino acid incorporation in isolated nuclei and in nuclei under conditions that should not permit protein import; and (3) the fact that nuclear translation would account for observations that are otherwise difficult to explain. Arguments against nuclear translation emphasize the absence (or low abundance) from nuclei of many translation factors; the likely inactivity of nascent ribosomes; and the loss of translation activity as nuclei are purified from contaminating cytoplasm. In our opinion, all of the experiments on nuclear translation published to date lack critical controls and, therefore, are not compelling; also, traditional mechanisms can explain the observations for which nuclear translation has been invoked. Thus, while we cannot rule out nuclear translation, in the absence of better supporting data we are reluctant to believe it occurs.

Separable roles for rent1/hUpf1 in altered splicing and decay of nonsense transcripts

The mechanism by which disruption of reading frame can influence pre-messenger RNA (pre-mRNA) processing is poorly understood. We assessed the role of factors essential for nonsense-mediated mRNA decay (NMD) in nonsense-mediated altered splicing (NAS) with the use of RNA interference (RNAi) in mammalian cells. Inhibition of rent1/hUpf1 expression abrogated both NMD and NAS of nonsense T cell receptor beta transcripts. In contrast, inhibition of rent2/hUpf2 expression did not disrupt NAS despite achieving comparable stabilization of nonsense transcripts. We also demonstrate that NAS and NMD are genetically separable functions of rent1/hUpf1. Additionally, rent1/hUpf1 enters the nucleus where it may directly influence early events in mRNA biogenesis. This provides compelling evidence that NAS relies on a component of the nonsense surveillance machinery but is not an indirect consequence of NMD.

Nonsense-associated altered splicing: a frame-dependent response distinct from nonsense-mediated decay

Nonsense-associated altered splicing (NAS) is a putative correction response that upregulates alternatively spliced transcripts that have skipped offending premature termination codons (PTCs). Here, we examined whether NAS has characteristics in common with nonsense-mediated decay (NMD), a surveillance mechanism that degrades PTC-bearing mRNAs. We discovered that although NAS shared the need for a Kozak AUG to define frame, it differed from NMD. NAS was not affected by depletion of the NMD protein hUPF2, and it functioned independently of RNA stabilization. We identified an alternatively spliced transcript acted upon by both NAS and NMD, indicating that these two mechanisms are not mutually exclusive. Our results suggest that NAS and NMD are distinct mechanisms despite being triggered by the same signal.