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Abstract In order to become bioactive, proteins must be translated and protected from aggregation during biosynthesis. The ribosome and molecular chaperones play a key role in this process. Ribosome-bound nascent chains (RNCs) of intrinsically disordered proteins and RNCs bearing a signal/arrest sequence are known to interact with ribosomal proteins. However, in the case of RNCs bearing foldable protein sequences, not much information is available on these interactions. Here, via a combination of chemical crosslinking and time-resolved fluorescence-anisotropy, we find that nascent chains of the foldable globin apoHmp_1–140interact with ribosomal protein L23 and have a freely-tumbling non-interacting N-terminal compact region comprising 63–94 residues. Longer RNCs (apoHmp_1–189) also interact with an additional yet unidentified ribosomal protein, as well as with chaperones. Surprisingly, the apparent strength of RNC/r-protein interactions does not depend on nascent-chain sequence. Overall, foldable nascent chains establish and expand interactions with selected ribosomal proteins and chaperones, as they get longer. These data are significant because they reveal the interplay between independent conformational sampling and nascent-protein interactions with the ribosomal surface. 

In order to become bioactive, proteins need to be biosynthesized and protected from aggregation during translation. The ribosome and molecular chaperones contribute to both tasks. While it is known that some ribosomal proteins (r-proteins) interact with ribosome-bound nascent chains (RNCs), specific interaction networks and their role within the ribosomal machinery remain poorly characterized and understood. Here, we find that RNCs of variable sequence and length (beyond the 1st C-terminal reside) do not modify the apparent stability of the peptidyl-transferase center (PTC) and r-proteins. Thus, RNC/r-protein interaction networks close to the PTC have no effect on the apparent stability of ribosome-RNC complexes. Further, fluorescence anisotropy decay, chemical-crosslinking and Western blots show that RNCs of the foldable protein apoHmp1-140 have an N-terminal compact region (6394 residues) and interact specifically with r-protein L23 but not with L24 or L29, at the ribosomal-tunnel exit. Longer RNCs bear a similar compact region and interact either with L23 alone or with L23 and another unidentified r-protein, or with molecular chaperones. The apparent strength of RNC/r-protein interactions does not depend on RNC sequence. Taken together, our findings show that RNCs encoding foldable protein sequences establish an expanding specific interaction network as they get longer, including L23, another r-protein and chaperones. Interestingly, the ribosome alone (i.e., in the absence of chaperones) provides indiscriminate support to RNCs bearing up to ca. 190 residues, regardless of nascent-chain sequence and foldability. In all, this study highlights the unbiased features of the ribosome as a powerful nascent-protein interactor.