DEGRADATION AND ENDOPLASMIC-RETICULUM RETENTION OF UNASSEMBLED ALPHA-SUBUNIT AND BETA-SUBUNIT OF NA,K-ATPASE CORRELATE WITH INTERACTION OF BIP

Assembly of alpha- and beta-subunits in the endoplasmic reticulum is a prerequisite for the structural and functional maturation of oligomer ic P-type ATPases. In Xenopus oocytes, overexpressed, unassembled alph a- and beta-subunits of Xenopus Na,K-ATPase are retained in the endopl asmic reticulum (ER) and are degraded with different kinetics, while u nassembled beta-subunits of gastric H,K-ATPase leave the ER. In this s tudy, we have investigated the role of the immunoglobulin-binding prot ein, BiP, in the folding, assembly, and ER retention of ATPase subunit s. We determined the primary sequence of Xenopus BiP and used polyclon al antibodies to examine the interaction with BiP of various wild type and mutant alpha- and beta-subunits overexpressed in Xenopus oocytes, Our results show that ER-retained, unassembled Na,K-ATPase beta-subun its, but not transport-competent H,K-ATPase beta-subunits, efficiently associate with BiP until assembly with alpha-subunits occurs. Further more, the kinetics of BiP interaction with unassembled wildtype and wi th mutant Na,K-ATPase beta-subunits parallels their respective stabili ty against cellular degradation. Finally, alpha-subunits that are over expressed in oocytes and are rapidly degraded and endogenous oocyte al pha-subunits that are stably expressed as individual assembly-competen t proteins also interact with oocyte or exogenous BiP, and the interac tion time correlates with the protein's stability. These data demonstr ate for the first time that BiP might be involved in a long term matur ation arrest and/or in the ER quality control of a multimembrane-spann ing protein and lend support for a universal chaperone function of BiP .