Biogenesis of α6β4 integrin in a human colonic adenocarcinoma cell line

The heterodimer α6β4 is a major integrin and the main laminin receptor in epithelia. The α6 integrin subunit is proteolytically cleaved, probably by furin, and glycosylated during its biosynthesis. In the present work, we have investigated the kinetics of the assembly process of α6β4 heterodimers in the colonic adenocarcinoma cell line HT29-D4. We demonstrate that the association of α6 and β4 precursors occurs within the ER, while the endoproteolytic cleavage of pro-α6 occurs later, probably in the trans-Golgi network. When pro-α6 was blocked within the ER by treatment with brefeldin A, its maturation processing was completely prevented without any consequence on its association with β4 subunit. Low temperature (20 °C) also blocked pro-α6 maturation, like brefeldin A, but in addition impaired the integrin assembly. Calnexin, an ER resident protein chaperone, was found to be associated with both the α6 and β4 subunit precursors. Our data suggest that calnexin might be responsible for the prolonged retention of pro-α6 within the ER compartment and for the defect of integrin subunit association observed at low temperature.