Signal sequences encode information for protein folding in the endoplasmic reticulum

Abstract

Roughly one-third of newly synthesized proteins enter into the endoplasmic reticulum (ER) via the Sec61 translocon. It is unclear how nascent chains bind correct chaperones and properly fold upon entering the ER lumen. We find that signal sequences harbor information to recruit specific chaperones for protein folding in the ER. Using a substrate-trapping proteomic approach, we identify that marginally hydrophobic signal sequences are transiently clogged at the Sec61 translocon, which recruits BiP chaperone through Sec63 to bind onto nascent chains. Surprisingly, this privileged BiP binding not only releases clogged nascent chains into the ER lumen but also prevent inappropriate interactions and promotes folding and maturation. Signal sequence swapping bypasses BiP-dependent unclogging and translocation, but the translocated nascent chain is terminally misfolded after binding the wrong chaperone in the ER lumen. Thus, signal sequence-dependent chaperone recruitment explains why signal sequences are paradoxically diverse and use multiple protein translocation pathways in cells.

A Comprehensive Subcellular Atlas of the Toxoplasma Proteome via hyperLOPIT Provides Spatial Context for Protein Functions

“…gondii and their apicomplexan orthologues for different endomembrane niches reveal statistically significant compositional differences between protein groups destined to different locations ( Figure 5 A and S6 ; Table S7 B). Signal peptide sequences are known to modulate the kinetics of Sec61 translocation and signal cleavage, both of which affect folding and chaperone recruitment, and in cases of ER stress, signal peptides can even selectively reroute proteins for cytosolic destruction ( Kang et al., 2006 ; Snapp et al., 2017 ; Sun et al., 2020 ). Our data imply that these or similar processes provide a level of sorting selection for apicomplexan secretory compartments at these early stages of protein synthesis.…”

Heat shock protein Hspa13 regulates endoplasmic reticulum and cytosolic proteostasis through modulation of protein translocation

“…Weaker signal sequences increase the reliance on post-translational translocation ( 15 , 112 ). Further investigations will need to establish the molecular mechanism by which Hspa13 regulates import as well as how this depends on the many other chaperones and stress factors that bind to and regulate the translocon ( 14 , 113 , 114 ), yielding a polydisperse distribution of translocon compositions ( 115 , 116 ) that impact import and maturation ( 117 ).…”

Hspa13 Regulates Endoplasmic Reticulum and Cytosolic Proteostasis Through Modulation of Protein Translocation

“…Weaker signal sequences increase the reliance on post-translational translocation (15, 110). Further investigations will need to establish the molecular mechanism by which Hspa13 regulates import, as well as how this depends on the many other chaperones and stress factors that bind to and regulate the translocon (14, 111, 112), yielding a polydisperse distribution of translocon compositions (113, 114) that impact import and maturation (115).…”