Cysteinyl-tRNA deacylation can be uncoupled from protein synthesis

Deep sequencing of tRNA's 3'-termini sheds light on CCA-tail integrity and maturation

The 3'-termini of tRNA are the point of amino acid linkage and thus crucial for their function in delivering amino acids to the ribosome and other enzymes. Therefore, to provide tRNA functionality, cells have to ensure the integrity of the 3'-terminal CCA-tail, which is generated during maturation by the 3'-trailer processing machinery and maintained by the CCA-adding enzyme. We developed a new tRNA sequencing method that is specifically tailored to assess the 3'-termini of E. coli tRNA. Intriguingly, we found a significant fraction of tRNAs with damaged CCA-tails under exponential growth conditions and, surprisingly, this fraction decreased upon transition into stationary phase. Interestingly, tRNAs bearing guanine as a discriminator base are generally unaffected by CCA-tail damage. In addition, we showed tRNA species-specific 3'-trailer processing patterns and reproduced in vitro findings on preferences of the maturation enzyme RNase T in vivo.

Peptide Channeling: The Key to MHC Class I Immunosurveillance?

MHC class I presentation of short peptides enables CD8⁺ T cell (T_CD8+) immunosurveillance of tumors and intracellular pathogens. A key feature of the class I pathway is that the immunopeptidome is highly skewed from the cellular degradome, indicating high selectivity of the access of protease-generated peptides to class I molecules. Similarly, in professional antigen-presenting cells, peptides from minute amounts of proteins introduced into the cytosol outcompete an overwhelming supply of constitutively generated peptides. Here, we propose that antigen processing is based on substrate channeling and review recent studies from the antigen processing and cell biology fields that provide a starting point for testing this hypothesis.