Biology subjects

Shan, S.-o.

Publications and source records attributed to Shan, S.-o..

4 recordsLinked to original sources

bioRxiv2025

NAC promotes co-translational folding at the ribosomal tunnel exit

The nascent polypeptide-associated complex (NAC) coordinates enzymatic modifications and membrane targeting of nascent chains during translation. While NACs function as a dynamic hub for other factors is well-established, its direct role in co-translational folding is unclear. By proteome-wide profiling NAC co-translational interactions in human cells, we found that NAC recognizes emerging segments enriched in hydrophobicity and -helical propensity, within folded domains of cytonuclear proteins. Single-molecule and structural analyses reveal that NAC, via its {beta}-barrel domain, dynamically interacts with nascent chains at the ribosomal tunnel exit and is capable of promoting on-pathway folding. Compartment-specific nascent chain interactions of NAC further elucidate its role in targeting to the endoplasmic reticulum and mitochondrial membrane protein biogenesis. Together, these findings show that NAC acts as a bona fide co-translational chaperone that facilitates early protein folding at the ribosomal tunnel exit, expanding its functional repertoire in protein biogenesis.

biochemistry↗

bioRxiv2025

A molecular switch in NAC prevents mitochondrial protein mistargeting by SRP

The nascent polypeptide-associated complex (NAC) co-translationally screens all nascent proteins and regulates their access to the signal recognition particle (SRP) to ensure the fidelity of protein targeting to the endoplasmic reticulum (ER). However, the mechanism by which NAC prevents the mistargeting of nascent mitochondrial proteins remains unclear. Here, we identified a molecular switch in NAC that allows its central barrel domain to adopt a stabilized conformation on ribosomes exposing a mitochondrial targeting sequence (MTS). Mutations of the MTS on the nascent chain or in the NAC switch region increases NAC barrel dynamics and reduces its binding to the ribosome. This leads to an impaired ability of NAC to prevent mistargeting by SRP and causes ER stress in human cells. Our work reveals how NAC detects nascent mitochondrial proteins early in translation and prevents their promiscuous access to SRP, elucidating the structural basis that underlies this role and providing novel insights into protein targeting fidelity with broader implications for cellular proteostasis.

biophysics↗

bioRxiv2024

Switchable client specificity in a dual functional chaperone coordinates light harvesting complex biogenesis

The proper assembly of light harvesting complexes (LHCs) is critical for photosynthesis and requires the biogenesis of light-harvesting chlorophyll a,b-binding proteins (LHCPs) to be coordinated with the biosynthesis of chlorophylls (Chl). The mechanism underlying this coordination is not well understood. Here we show that a conserved molecular chaperone in chloroplasts, cpSRP43, provides a molecular thermostat that helps maintain this coordination. cpSRP43 undergoes a conformational rearrangement between a well-folded closed state and a partially disordered open state. Closed cpSRP43 is dedicated to the de novo biogenesis of LHCPs, whereas open cpSRP43 protects multiple Chl biosynthesis enzymes from heat-induced destabilization. Rising temperature shifts cpSRP43 to the open state and thus enables it to protect Chl biosynthesis enzymes that are heat-destabilized. Our results reveal the molecular basis of a post-translational mechanism for the thermo-adaptation of LHC biogenesis. They also demonstrate how an ATP-independent chaperone uses conformational dynamics to switch its activity and client selectivity, thereby adapting to different proteostatic demands under shifting environmental conditions. TeaserA thermo-switchable molecular chaperone helps coordinate light harvesting complex assembly during photosynthesis.

biochemistry↗

bioRxiv2020

Receptor compaction and GTPase movements drive cotranslational protein translocation

Signal recognition particle (SRP) is a universally conserved targeting machine that couples the synthesis of ~30% of the proteome to their proper membrane localization1,2. In eukaryotic cells, SRP recognizes translating ribosomes bearing hydrophobic signal sequences and, through interaction with SRP receptor (SR), delivers them to the Sec61p translocase on the endoplasmic reticulum (ER) membrane1,2. How SRP ensures efficient and productive initiation of protein translocation at the ER is not well understood. Here, single molecule fluorescence spectroscopy demonstrates that cargo-loaded SRP induces a global compaction of SR, driving a >90 [A] movement of the SRP*SR GTPase complex from the vicinity of the ribosome exit, where it initially assembles, to the distal site of SRP. These rearrangements bring translating ribosomes near the membrane, expose conserved Sec61p docking sites on the ribosome and weaken SRPs interaction with the signal sequence on the nascent polypeptide, thus priming the translating ribosome for engaging the translocation machinery. Disruption of these rearrangements severely impairs cotranslational protein translocation and is the cause of failure in an SRP54 mutant linked to severe congenital neutropenia. Our results demonstrate that multiple largescale molecular motions in the SRP*SR complex are required to drive the transition from protein targeting to translocation; these post-targeting rearrangements provide potential new points for biological regulation as well as disease intervention.

biochemistry↗