Intra-membrane client recognition potentiates the chaperone functions of Calnexin

Abstract

One third of the human proteome are membrane proteins. They are particularly vulnerable to misfolding, often requiring assistance by molecular chaperones. Calnexin (CNX), one of the most abundant ER chaperones, plays an important role in membrane protein biogenesis and engages clients via its sugar-binding lectin domain.Using mass spectrometric analyses, we show that Calnexin (CNX) interacts with a large number of non-glycosylated membrane proteins, suggesting additional binding modes. We find that misfolded membrane proteins are preferentially bound by CNX and that CNX uses its single transmembrane domain (TMD) for client recognition. Combining experimental and computational approaches, we systematically dissect signatures for intramembrane client recognition by CNX and identify sequence motifs within the CNX TMD region that mediate client binding. Building on this, we show that intramembrane client binding potentiates the chaperone functions of CNX.Together, this study reveals a widespread role of CNX 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.

Calnexin reveals a sugar‐free taste within the lipid bilayer

“…An earlier study identifying an interaction between calnexin and proteolipid protein (Swanton et al, 2003) had led to the proposal of a glycan-independent role of the calnexin transmembrane domain (TMD) in binding clients within the membrane. The new study by the Feige group now further analyzed this carbohydrate-independent role of the calnexin transmembrane region in binding and assisting the intramembrane domains of maturing membrane protein clients (Bloemeke et al, 2022).…”

“…Rhodopsin was degraded more rapidly in cells expressing the calnexin Y/T/L mutant, suggesting that the intramembrane interaction was important for rhodopsin maturation and stability. Based on these results, Bloemeke et al (2022) propose a model in which calnexin interacts with transmembrane protein substrates through: (i) the calnexin TMD and a substrate TMD; (ii) an N-glycan and the calnexin lectin domain; or (iii) using both modes (Fig 1). These results greatly expand the potential clientele of calnexin by including non-glycosylated membrane proteins.…”