Stimulation of Ribosomal Frameshifting by Cotranslational Membrane Protein Folding and Misfolding

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ABSTRACT The proteostasis network relies on numerous feedback mechanisms to strike a balance between the rates of protein synthesis and degradation, which is crucial for the maintenance of protein homeostasis. Proper tuning of the rate of protein synthesis is also critical for the fidelity of cotranslational protein folding, which requires coordination between the ribosome and various molecular chaperones. This translational regulation is especially important for the fidelity of membrane protein (MP) biosynthesis, as the disruption of translational dynamics appears to coincide with cotranslational misfolding and premature degradation. Nevertheless, it is currently unclear how the translational machinery detects and responds to the cotranslational MP misfolding. In a recent study of the topological properties of the Sindbis virus (SINV) structural polyprotein, our team found that the translocon-mediated membrane integration of the nascent polypeptide stimulates ribosomal frameshifting and the premature termination of translation. This work revealed that cotranslational (mis)folding can alter translation through programmed ribosomal frameshifting (PRF), which is typically viewed as an RNA-mediated translational recoding mechanism. In the following, we outline evidence suggesting translocon-mediated PRF occurs during the translation of many human MPs, including several misfolding-prone MPs such as the cystic fibrosis transmembrane conductance regulator (CFTR). We provide multiple lines of evidence that demonstrate that PRF can occur at several “checkpoints” during CFTR synthesis, and show that a pathogenic mutation known to induce cotranslational misfolding (ΔF508) stimulates ribosomal frameshifting and the premature termination of CFTR translation. Based on these findings, we hypothesize that PRF sites allow the ribosome to tune the processivity of translation in response to conformational transitions in the nascent chain. To test this hypothesis, we will assess how mutations and small molecules that alter cotranslational CFTR folding impacts the processivity of translation at each PRF site. To gain structural insights into this ribosomal frameshifting mechanism, we will also extend our studies on the SINV structural polyprotein. To map the sequence constraints of translocon-mediated PRF, we measured the effects of 2,003 mutations on the efficiency of ribosomal frameshifting by deep mutational scanning. Our preliminary results reveal several structural features that appear to be critical for PRF, including a putative lipid-binding face within a nascent transmembrane domain and a helical segment within the ribosomal exit tunnel. To determine how these structural features induce PRF, we propose a novel fusion of molecular modeling, cellular biochemistry, and virology experiments to elucidate these structural features. Finally, we will leverage these insights to develop sequence-based energetic predictions for the efficiency of PRF within integral MPs. We will also characterize putative PRF sites in several disease-linked MPs in order to validate these findings and explore the potential role of PRF in MP homeostasis. Together, these investigations will provide fundamental insights into a novel cotranslational feedback mechanism and the molecular basis of disease.