Biogenesis of alpha6beta4 integrin in a human colonic adenocarcinoma cell line involvement of calnexin.

The heterodimer alpha6beta4 is a major integrin and the main laminin receptor in epithelia. The alpha6 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 alpha6beta4 heterodimers in the colonic adenocarcinoma cell line HT29-D4. We demonstrate that the association of alpha6 and beta4 precursors occurs within the ER, while the endoproteolytic cleavage of pro-alpha6 occurs later, probably in the trans-Golgi network. When pro-alpha6 was blocked within the ER by treatment with brefeldin A, its maturation processing was completely prevented without any consequence on its association with beta4 subunit. Low temperature (20 degrees C) also blocked pro-alpha6 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 alpha6 and beta4 subunit precursors. Our data suggest that calnexin might be responsible for the prolonged retention of pro-alpha6 within the ER compartment and for the defect of integrin subunit association observed at low temperature.