A Moment's Hesitation

HIGHLIGHTS

PROTEIN SYNTHESIS IN BRIEF

A moment’s APOPTOSIS hesitation Cytochrome c binds to inositol (1,4,5) trisphosphate receptors, amplifying calcium-dependent apoptosis. Nature has developed quality-control Boehning, D. et al. Nature Cell Biol. 9 Nov 2003 (DOI: 10.1038/ncb1063) mechanisms to ensure accurate pro- That small concentrations of Ca2+ can inhibit inositol-1,4,5- tein synthesis. In a report in Molecular trisphosphate receptor (Ins(1,4,5)P R) function is a well-known Cell,Hayes and Sauer now show that 3 feedback mechanism to prevent excess Ca2+ release. Boehning and when a bacterial ribosome pauses dur- colleagues now show that physiological quantities of cytochrome c ing polypeptide synthesis, the messen- can block this Ca2+-mediated inhibition of Ins(1,4,5)P R function. ger RNA that is being translated is By growing cells in the presence of 3 Early in apoptosis, cytochrome c released from mitochondria cleaved at the codon that occupies the a proline analogue instead of proline, translocates to the endoplasmic reticulum and binds Ins(1,4,5)P R. ribosomal A site. They propose that both ribosomal pausing and polypep- 3 This sensitizes Ins(1,4,5)P R to increased Ca2+ release, causing a this quality-control mechanism — tide tagging were reduced, which cor- 3 large cytochrome c release that amplifies the apoptotic signal. which they refer to as A-site mRNA related with a reduction in the level of cleavage — together with another truncated transcript. The same effect TELOMERES translational quality-control system on A-site mRNA cleavage was shown (the tmRNA system), provides a way when protein release factor 1 (RF1), DNA damage foci at dysfunctional telomeres. of reducing translational errors and which is known to reduce ribosomal therefore the production of harmful pausing, was overproduced. This led Takai, H. et al. Curr. Biol. 13, 1549–1556 (2003) polypeptides. the authors to conclude that both A DNA damage checkpoint response in telomere- Ribosomal pausing can occur at translation and ribosome pausing are initiated senescence. rare codons or stop codons because required for A-site mRNA cleavage. the cognate transfer RNA or protein- Indeed, when Hayes and Sauer did the d’Adda di Fagagna, F. et al. Nature 5 Nov 2003 (DOI: 10.1038/nature02118) release factor, which is needed for reverse experiment, using conditions The groups of de Lange and Jackson now show that dysfunctional translation termination, is scarce. that increased ribosomal pausing, the telomeres activate a DNA-damage-checkpoint response. de Lange The authors used a version of the level of truncated mRNA increased. and colleagues uncapped telomeres by inhibiting the duplex Escherichia coli ybeL gene, which Next, the authors tested the E. coli TTAGGG repeat binding factor (TRF2), which is essential for encodes a protein that ends with two RelE toxin, which is known to be a telomere protection. Known DNA-damage-response factors, proline residues (YbeL-PP), to study its mediator of A-site mRNA cleavage, including 53BP1, γ-H2AX, Rad17, ATM and Mre11, became effects — the carboxy-terminal proline and a range of other bacterial toxins associated with the uncapped dysfunctional telomeres at ‘telomere residue of YbeL-PP causes inefficient for their possible involvement in ybeL- dysfunction-induced foci’ (TIFs). And inhibiting the upstream translation termination, which results PP mRNA cleavage in vivo.None of protein kinase activators of the DNA-damage checkpoint reduced in ribosome pausing. Northern-blot the toxin-deleted strains had any effect the accumulation of 53BP1. Using human diploid fibroblast (HDF) analysis of cellular RNA showed a on A-site mRNA cleavage, as shown by cell lines that undergo senescence as a result of telomere shortening, truncated ybeL-PP mRNA that was northern blots and by assaying for Jackson and colleagues detected a ‘senescence-associated DNA close in size to a control ybeL tran- polypeptide tagging. The ribosomal damage focus’ (SDF) of γ-H2AX colocalized with 53BP1, MDC1, script, which ends after the second base alarmone (p)ppGpp (a guanine NBS1 and SMC1 pS966. This telomere-initiated senescence triggers of the stop codon. More precise map- nucleotide that is synthesized when a complete DNA-damage response — dysfunctional telomeres ping confirmed that ybeL-PP mRNA ribosomes stall) also does not seem to contribute directly to this response. Accumulation of the DNA- was cleaved at, or near, the stop codon. have a role in either the cleavage or damage-response factors was also seen when TRF2 was inhibited in Interestingly, Hayes and Sauer tagging process. immortalized human fibroblasts. found that tmRNA (a specialized RNA As is often the case in science, new that ensures the tagging of polypep- findings give rise to further questions PROTEIN ASSEMBLY tides for degradation) is not essential — the nuclease that is responsible for for A-site mRNA cleavage.Yet,when A-site mRNA cleavage is still elusive, An essential role of Sam50 in the protein sorting and tmRNA is present, it ensures that the and the authors speculate that the assembly machinery of the mitochondrial outer polypeptide at the pausing ribosome is ribosome itself might mediate this membrane. tagged for degradation — thereby cleavage reaction. In addition, whether Kozjak, V. et al. J. Biol. Chem. 15 Oct 2003 [epub ahead of print] linking A-site mRNA cleavage to the a similar mechanism exists in other tmRNA quality-control system. bacteria, archaebacteria or eukaryotes Kozjak and colleagues report the identification of Sam50, which By introducing an amber stop is, for the moment, anyone’s guess. associates with Mas37 to form the Saccharomyces cerevisiae codon in the ybeL-PP gene, which pre- Arianne Heinrichs mitochondrial outer membrane sorting and assembly machinery vents efficient translation of the ybeL- References and links (SAM complex). Using an in organello assembly assay, Sam50 was PP transcript, the level of truncated ORIGINAL RESEARCH PAPER Hayes, C. S. & shown to be crucial for the assembly of β-barrel proteins of the mRNA was reduced significantly. So, Sauer, R. T. Cleavage of the A site mRNA codon outer membrane, such as Tom40 and porin. In addition, Sam50 is during ribosome pausing provides a mechanism for translation is clearly important for translational quality control. Mol. Cell 12, 903–911 only the second mitochondrial outer membrane protein that is A-site mRNA cleavage. (2003) known to be essential for cell viability.

NATURE REVIEWS | MOLECULAR CELL BIOLOGY VOLUME 4 | DECEMBER 2003 | 911