1. Institute of Molecular Biology and Biophysics, ETH Zurich, 8093 Zurich, Switzerland
2. Zentrum für Molekulare Biologie (ZMBH), Universität Heidelberg, Im Neuenheimer Feld 282, 69120 Heidelberg, Germany
3. Novartis Institutes for BioMedical Research, Inc., 500 Technology Square, Cambridge, Massachusetts, 02139, USA
4. Present address: Solvias AG, Postfach, 4002 Basel, Switzerland.

Correspondence to: Nenad Ban¹ Correspondence and requests for materials should be addressed to N.B. (Email: ban@mol.biol.ethz.ch).

Abstract

Messenger-RNA-directed protein synthesis is accomplished by the ribosome^{1, 2, 3}. In eubacteria, this complex process is initiated by a specialized transfer RNA charged with formylmethionine (tRNA^fMet)^{4, 5, 6}. The amino-terminal formylated methionine of all bacterial nascent polypeptides blocks the reactive amino group to prevent unfavourable side-reactions and to enhance the efficiency of translation initiation^{7, 8}. The first enzymatic factor that processes nascent chains is peptide deformylase (PDF)^{5, 9, 10, 11}; it removes this formyl group as polypeptides emerge from the ribosomal tunnel^{12, 13} and before the newly synthesized proteins can adopt their native fold, which may bury the N terminus. Next, the N-terminal methionine is excised by methionine aminopeptidase¹⁴. Bacterial PDFs are metalloproteases sharing a conserved N-terminal catalytic domain. All Gram-negative bacteria, including Escherichia coli, possess class-1 PDFs characterized by a carboxy-terminal -helical extension¹⁵. Studies focusing on PDF as a target for antibacterial drugs^{14, 16} have not revealed the mechanism of its co-translational mode of action despite indications in early work that it co-purifies with ribosomes¹⁷. Here we provide biochemical evidence that E. coli PDF interacts directly with the ribosome via its C-terminal extension. Crystallographic analysis of the complex between the ribosome-interacting helix of PDF and the ribosome at 3.7 Å resolution reveals that the enzyme orients its active site towards the ribosomal tunnel exit for efficient co-translational processing of emerging nascent chains. Furthermore, we have found that the interaction of PDF with the ribosome enhances cell viability. These results provide the structural basis for understanding the coupling between protein synthesis and enzymatic processing of nascent chains, and offer insights into the interplay of PDF with the ribosome-associated chaperone trigger factor.

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