KDEL proteins are found on the surface of NG108-15 cells

Although KDEL proteins are primarily localized to the endoplasmic reticulum (ER), we have employed surface biotinylation method to demonstrate that th e KDEL proteins calreticulin (Crt), protein disulfide isomerase (PDI) and t he 78-kDa glucose regulated protein (GRP78) are found on the surface of the NG108-15 cell line. In contrast, the 94-kDa glucose regulated protein (GRP 94), another KDEL protein, is not found on the cell surface. Calnexin (Cnx) , a type-1 integral transmembrane ER protein which is partially homologous to Crt but lacks the KDEL sequence, is not detected on the cell surface eit her. While only small amounts of the total GRP78, PDI and Crt molecules exi st on the cell surface at steady state, a significant fraction of the newly synthesized molecules are transported to the cell surface and transport of these proteins is inhibited by treatment with brefeldin A. The surface GRP 78 contains the KDEL sequence. On the cell surface, GRP78, PDI and Crt asso ciate with other proteins and form complexes of different sizes. Surface Cr t is found to be essential far the neurite formation when NG108-15 cells ar e induced to differentiate using dibutyryl cAMP. (C) 1999 Elsevier Science B.V. All rights reserved.