N-linked glycans direct the cotranslational folding pathway of influenza hemagglutinin.

For proteins that traverse the secretory pathway, folding commences cotranslationally upon translocation into the endoplasmic reticulum. In this study, we have comprehensively analyzed the earliest maturation steps of the model glycoprotein influenza hemagglutinin (HA). These steps include cleavage of the signal sequence, glycosylation, binding by the chaperones calnexin and calreticulin, and the oxidoreductase ERp57, and oxidation. Our results show that the molecular choreography of the nascent HA chain is largely directed by multiple glycans that are strategically placed to elicit the binding of lectin chaperones. These chaperones are recruited to specific nascent chain locations to regulate and facilitate glycoprotein folding, thereby suggesting that the positioning of N-linked glycans in critical regions has evolved to optimize the folding process in the cell.