Integrin subunits beta 1C-1 and beta 1C-2 expressed in GD25T cells are retained and degraded intracellularly rather than localised to the cell surface

We have previously identified the integrin beta 1C-2 rand characterised the distribution of beta 1C-1 and beta 1C-2 transcripts in various cell lines and normal cells. In this study we have investigated the expression of the two beta 1C-variants in integrin beta 1 deficient mouse GD25T cells. After stable transfection of the GD25T cells with cDNAs coding for beta 1A, beta 1C-1 and beta 1C-2, the cell surface expression of the beta 1C-1 and beta 1 C-2 variants was found to be very low while the beta 1A variant was express ed at high levels. Northern blot analysis showed that the level of beta 1-t ranscript in the beta 1C-1 and beta 1C-2 clones was equal or higher than in the beta 1A clones. Metabolic labelling and deglycosylation by endoglycosi dase H treatment clearly demonstrated that the majority of the beta 1C-1 an d beta 1C-2 chains did not become maturely glycosylated, nor did they dimer ise with a subunits. After 20 hours of chase, the labelled beta 1C-1 and be ta 1C-2 chains had been gradually degraded, whereas immature beta 1A was co nverted into the maturely glycosylated form during the same period of time. Immunostaining showed intracellular beta 1 localisation in the beta 1C-1 a nd beta 1C-2 expressing clones, while in the beta 1A expressing clones the beta 1 chains were mainly localised to focal adhesion sites and along fibro nectin fibres, Taken together, we have shown that expression of both integr in beta 1C-1 and beta 1C-2 in GD25T cells result in very low cell surface e xpression compared with the normal beta 1A isoform. Instead, both beta 1C-1 and beta 1C-2 chains remain in the endoplasmic reticulum until they are in tracellularly degraded.