Calreticulin is transported to the surface of NG108-15 cells where it forms surface patches and is partially degraded in an acidic compartment

Although calreticulin (Crt) is primarily localized to the endoplasmic retic ulum (ER), our results using biotinylation and immunocytochemical methods i ndicate that a small but significant amount of Crt is present and forms lar ge patches on the surface of NG108-15 cells (a mouse neuroblastoma-rat glio ma hybrid cell line). S-35-labelled Crt molecules begin to reach the cell s urface after only 10 min of labelling and disappear slowly from the cell su rface, After 4 hr of labelling, approximately half of the newly synthesized Crt molecules are on the cell surface. We believe that some Crt molecules may escape from the KDEL receptor-mediated salvage pathway as they are synt hesized and proceed through the secretory pathway to the cell surface. Immu noprecipitation from the culture medium shows that Crt is not released from the cell surface to the medium, suggesting tight binding to surface molecu les, NH4Cl can block the degradation of Crt; therefore, Crt is presumably d egraded in the lysosome pathway. However, blockage of the disappearance of surface Crt is less efficient than that of internal Crt. This suggests that the disappearance of Crt from the cell surface may not be due solely to it s degradation, but may reflect transport into another cell compartment such as the ER. (C) 1999 Wiley-Liss, Inc.