Intramembrane client recognition potentiates the chaperone functions of calnexin.

One-third of the human proteome is comprised of membrane proteins, which are particularly vulnerable to misfolding and often require folding assistance by molecular chaperones. Calnexin (CNX), which engages client proteins via its sugar-binding lectin domain, is one of the most abundant ER chaperones, and plays an important role in membrane protein biogenesis. Based on mass spectrometric analyses, we here show that calnexin interacts with a large number of nonglycosylated membrane proteins, indicative of additional nonlectin binding modes. We find that calnexin preferentially bind misfolded membrane proteins and that it uses its single transmembrane domain (TMD) for client recognition. Combining experimental and computational approaches, we systematically dissect signatures for intramembrane client recognition by calnexin, and identify sequence motifs within the calnexin TMD region that mediate client binding. Building on this, we show that intramembrane client binding potentiates the chaperone functions of calnexin. Together, these data reveal a widespread role of calnexin client recognition in the lipid bilayer, which synergizes with its established lectin-based substrate binding. Molecular chaperones thus can combine different interaction modes to support the biogenesis of the diverse eukaryotic membrane proteome.

The molecular basis of chaperone-mediated interleukin 23 assembly control.

The functionality of most secreted proteins depends on their assembly into a defined quaternary structure. Despite this, it remains unclear how cells discriminate unassembled proteins en route to the native state from misfolded ones that need to be degraded. Here we show how chaperones can regulate and control assembly of heterodimeric proteins, using interleukin 23 (IL-23) as a model. We find that the IL-23 α-subunit remains partially unstructured until assembly with its ß-subunit occurs and identify a major site of incomplete folding. Incomplete folding is recognized by different chaperones along the secretory pathway, realizing reliable assembly control by sequential checkpoints. Structural optimization of the chaperone recognition site allows it to bypass quality control checkpoints and provides a secretion-competent IL-23α subunit, which can still form functional heterodimeric IL-23. Thus, locally-restricted incomplete folding within single-domain proteins can be used to regulate and control their assembly.

Publisher Correction: A network of chaperones prevents and detects failures in membrane protein lipid bilayer integration.

The original version of this Article contained errors in Fig. 1 and Supplementary Fig. 3. In Fig. 1, the labels indicating the Cx32wt constructs in panels d and e were incorrectly shifted with respect to the relevant western blot lanes. In Supplementary Fig. 3, numbers of unique peptides and % sequence coverage were incorrectly reported as being for wt and L90H separately, and should refer to wt and L90H combined. These errors have been corrected in the PDF and HTML versions of the Article.

A network of chaperones prevents and detects failures in membrane protein lipid bilayer integration.

A fundamental step in membrane protein biogenesis is their integration into the lipid bilayer with a defined orientation of each transmembrane segment. Despite this, it remains unclear how cells detect and handle failures in this process. Here we show that single point mutations in the membrane protein connexin 32 (Cx32), which cause Charcot-Marie-Tooth disease, can cause failures in membrane integration. This leads to Cx32 transport defects and rapid degradation. Our data show that multiple chaperones detect and remedy this aberrant behavior: the ER-membrane complex (EMC) aids in membrane integration of low-hydrophobicity transmembrane segments. If they fail to integrate, these are recognized by the ER-lumenal chaperone BiP. Ultimately, the E3 ligase gp78 ubiquitinates Cx32 proteins, targeting them for degradation. Thus, cells use a coordinated system of chaperones for the complex task of membrane protein biogenesis, which can be compromised by single point mutations, causing human disease.

A DNA-dependent protease involved in DNA-protein crosslink repair.

Toxic DNA-protein crosslinks (DPCs) arise by ionizing irradiation and UV light, are particularly caused by endogenously produced reactive compounds such as formaldehyde, and also occur during compromised topoisomerase action. Although nucleotide excision repair and homologous recombination contribute to cell survival upon DPCs, hardly anything is known about mechanisms that target the protein component of DPCs directly. Here, we identify the metalloprotease Wss1 as being crucial for cell survival upon exposure to formaldehyde and topoisomerase 1-dependent DNA damage. Yeast mutants lacking Wss1 accumulate DPCs and exhibit gross chromosomal rearrangements. Notably, in vitro assays indicate that substrates such as topoisomerase 1 are processed by the metalloprotease directly and in a DNA-dependent manner. Thus, our data suggest that Wss1 contributes to survival of DPC-harboring cells by acting on DPCs proteolytically. We propose that DPC proteolysis enables repair of these unique lesions via downstream canonical DNA repair pathways.