Protein complexes are pivotal to most cellular processes. Emerging evidence indicating dimer assembly by pairs of ribosomes suggests yet unknown folding mechanisms involving two nascent chains. Here, we show that co-translational ribosome pairing allows their nascent chains to ‘chaperone each other’, thus enabling the formation of coiled-coil homodimers from subunits that misfold individually. We developed an integrated single-molecule fluorescence and force spectroscopy approach to probe the folding and assembly of two nascent chains extending from nearby ribosomes, using the intermediate filament lamin as a model system. Ribosome proximity during early translation stages is found to be critical: when interactions between nascent chains are inhibited or delayed, they become trapped in stable misfolded states that are no longer assembly-competent. Conversely, early interactions allow the two nascent chains to nucleate native-like quaternary structures that grow in size and stability as translation advances. We conjecture that protein folding mechanisms enabled by ribosome cooperation are more broadly relevant to intermediate filaments and other protein classes.

Conformational control of nascent chains is poorly understood. Chaperones are known to stabilize, unfold, and disaggregate polypeptides away from the ribosome. In comparison, much less is known about the elementary conformational control mechanisms at the ribosome. Yet, proteins encounter major folding and aggregation challenges during translation. Here, using selective ribosome profiling and optical tweezers with correlated single-molecule fluorescence, with dihydrofolate reductase (DHFR) as a model system, we show that the Escherichia coli chaperone trigger factor (TF) accelerates nascent chain folding. TF scans nascent chains by transient binding events, and then locks into a stable binding mode as the chain collapses and folds. This interplay is reciprocal: TF binding collapses nascent chains and stabilizes partial folds, while nascent chain compaction prolongs TF binding. Ongoing translation controls these cooperative effects, with TF-accelerated folding depending on the emergence of a peptide segment that is central to the core DHFR beta-sheet. The folding acceleration we report here impacts processes that depend on folding occurring cotranslationally, including cotranslational protein assembly, protein aggregation, and translational pausing, and may be relevant to other domains of life.

Proteins commonly fold co-translationally at the ribosome, while the nascent chain emerges from the ribosomal exit tunnel. Protein domains that are sufficiently small can even fold while still located inside the tunnel. However, the effect of the tunnel on the folding dynamics of these domains is not well understood. Here, we combine optical tweezers with single-molecule FRET and molecular dynamics simulations to investigate folding of the small zinc-finger domain ADR1a inside and at the vestibule of the ribosomal tunnel. The tunnel is found to accelerate folding and stabilize the folded state, reminiscent of the effects of chaperonins. However, a simple mechanism involving stabilization by confinement does not explain the results. Instead, it appears that electrostatic interactions between the protein and ribosome contribute to the observed folding acceleration and stabilization of ADR1a.