Practice question

Stabilization of unfolded secretory proteins inside endoplasmic reticulum oxidizing environment depends on lectin chaperone system that monitors glycosylation status and provides time for folding. Calreticulin, soluble paralog of membrane-bound calnexin sharing lectin domain but lacking transmembrane anchor, resides in ER lumen at millimolar concentration. It binds specifically monoglucosylated Glc1Man9GlcNAc2 N-glycans generated after glucosidase I and II trimming of precursor added en bloc. Its long proline-rich P-domain arm extends to recruit oxidoreductase ERp57 forming mixed disulfides with client to catalyze correct disulfide pairing while holding client soluble preventing aggregation. Substrates include MHC class I heavy chains before beta2-microglobulin association, influenza hemagglutinin, CFTR and many hormones. Unlike cytosolic Hsp90, KDEL receptor that retrieves escaped ER proteins via COPI, or Rab GTPases regulating vesicle docking and tethering, calreticulin directly holds folding intermediates through glycan recognition and acts also as major calcium buffer storing up to fifty percent of ER calcium via acidic C-domain, linking calcium homeostasis to proteostasis and antigen presentation efficiency and developmental processes. Additional coordination with cellular stress pathways ensures fidelity, prevents aggregation, and links trafficking to growth control and proteostasis maintenance across diverse cell types and developmental stages.