Induction of glucose-regulated proteins in Xenopus laevis A6 cells

We have characterized the induction of glucose-regulated proteins (GRPs) in Xenopus laevis A6 cells, a kidney epithelial cell line. Exposure of A6 cells to medium in which 2-deoxyglucose replaced galactose resulted in enhanced synthesis of two proteins at 78 and 98 kd. The 78 kd protein was determined by two-dimensional PAGE to consist of two isoelectric variants with pls of 5.3 and 5.2 whereas the 98 kd protein resolved into a single spot with a pl of 5.1. The 78 kd protein cross-reacted with antiserum against chicken GRP78 (glucose-regulated protein), suggesting that the Xenopus protein shares homology with a previously characterized GRP. This was supported by the finding that a rat GRP78 probe hybridized with a 2-deoxyglucose-inducible mRNA. Synthesis of the two proteins was also induced by tunicamycin, 2-deoxygalactose, and dithiothreitol. However, the GRPs were not induced by glucosamine or calcium ionophore A23187 at concentrations and exposure periods that have previously been shown to elicit a GRP response in mammalian and avian cells. Enhanced synthesis of the two GRPs by 2-deoxyglucose was transient, reaching maximal levels by 12-24 h and decreasing to near control levels by 48 h. Removal of the stress at the point of peak synthesis resulted in decreased synthesis of both proteins within 6 h and a return to control levels within 24 h of recovery. These data suggest that Xenopus cells have a GRP response that is similar, but not identical, to that found in mammalian cells.