Biogenesis of alpha 6 beta 4 integrin in a human colonic adenocarcinoma cell line - Involvement of calnexin

The heterodimer alpha 6 beta 4 is a major integrin and the main laminin rec eptor in epithelia. The alpha 6 integrin subunit is proteolytically cleaved , probably by furin, and glycosylated during its biosynthesis. In the prese nt work, we have investigated the kinetics of the assembly process of alpha 6 beta 4 heterodimers in the colonic adenocarcinoma cell line HT29-D4. We demonstrate that the association of alpha 6 and beta 4 precursors occurs wi thin the ER, while the endoproteolytic cleavage of pro-alpha 6 occurs later , probably in the trans-Golgi network. When pro-alpha 6 was blocked within the ER by treatment with brefeldin A, its maturation processing was complet ely prevented without any consequence on its association with beta 4 subuni t. Low temperature (20 degrees C) also blocked pro-alpha 6 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 alp ha 6 and beta 4 subunit precursors. Our data suggest that calnexin might be responsible for the prolonged retention of pro-alpha 6 within the ER compa rtment and for the defect of integrin subunit association observed at low t emperature.