pH-induced conversion of the transport lectin ERGIC-53 triggers glycoprotein release

The recycling mannose lectin ERGIC-53 operates as a transport receptor by mediating efficient endoplasmic reticulum (ER) export of some secretory glycoproteins. Binding of cargo to ERGIC-53 in the ER requires Ca2+. Cargo release occurs in the ERGIC, but the molecular mechanism is unknown. Here we report efficient binding of purified ERGIC-53 to immobilized mannose at pH 7.4, the pH of the ER, but not at slightly lower pH. pH sensitivity of the lectin was more prominent when Ca2+ concentrations were low. A conserved histidine in the center of the carbohydrate recognition domain was required for lectin activity suggesting it may serve as a molecular pH/Ca2+ sensor. Acidification of cells inhibited the association of ERGIC-53 with the known cargo cathepsin Z-related protein and dissociation of this glycoprotein in the ERGIC was impaired by organelle neutralization that did not impair the transport of a control protein. The results elucidate the molecular mechanism underlying reversible lectin/cargo interaction and establish the ERGIC as the earliest low pH site of the secretory pathway.