A SIGNAL FOR ENDOPLASMIC-RETICULUM RETENTION LOCATED AT THE CARBOXYL-TERMINUS OF THE PLASMA-MEMBRANE CA2-ATPASE ISOFORM 4CI()

The plasma membrane Ca2+-ATPase isoform 4b (PMCA4CI) with truncations in the cytoplasmically exposed COOH-terminal tail was expressed in COS and HeLa cells and in Sf9 cells using the baculovirus system. The tru ncated protein terminating with the acidic sequence Glu(1067)-Arg(1087 ) was retained within the endoplasmic reticulum (ER), whereas mutants lacking this sequence or having it at a distance from the COOH terminu s were delivered to the plasma membrane. Although the truncated protei n retained in the endoplasmic reticulum was still able to form a Ca2+- dependent phosphoenzyme, it underwent partial degradation, Substitutio n of glutamic and aspartic residue(s) in the acidic region promoted re scue of the protein to the plasma membrane. The results suggest that t he sequence Glu(1067)-Arg(1087) encodes a mashed signal for ER retenti on and for the degradation of the protein. However, its presence at th e COOH terminus was not sufficient to induce ER-retention and degradat ion; when the sequence was attached to the full-length PMCA protein, n ormal plasma delivery was observed. Evidently, ER retention and degrad ation required the presence of the sequence in its specific location w ithin the PMCA structure. The degradation of the protein retained in t he endoplasmic reticulum occurred through the proteolytic attack at cy toplasmically exposed residues (amino acid sequence 720-750) by a cyto plasmic PEST sequence-related protease different from calpain.