The endoplasmic reticulum calcium-binding protein of 55 kDa is a novel EF-hand protein retained in the endoplasmic reticulum by a carboxyl-terminal His-Asp-Glu-Leu motif.
We have identified a new human Ca(2+)-binding protein that is specifically localized in the endoplasmic reticulum (ER). The protein is termed ERC-55, i.e. ER calcium-binding protein of 55 kDa. ERC-55 is a single copy gene and is encoded by an approximately 1900-base mRNA, which shows a ubiquitous expression pattern. The ERC-55 protein comprises an amino-terminal signal sequence followed by six copies of the EF-hand Ca2+ binding motif. Ca2+ binding was demonstrated directly for recombinant ERC-55 using the 45Ca2+ overlay technique. The carboxyl-terminal sequence His-Asp-Glu-Leu (HDEL) is required for retention of ERC-55 in the ER. Deletion of HDEL results in slow secretion into the medium. In pulse-chase experiments, approximately 50% of the HDEL deletion mutant is secreted, whereas no detectable secretion is observed with the wild-type protein. This represents the first example of an endogenous human protein that is retained in the ER by an HDEL rather than Lys-Asp-Glu-Leu (KDEL) carboxyl-terminal tetrapeptide. Comparative sequence analysis indicates that ERC-55, together with the recently identified protein reticulocalbin (Ozawa and Muramatsu, 1993), constitute a new subfamily of the EF-hand superfamily of Ca(2+)-binding proteins that are specifically located in the ER.
European Molecular Biology Laboratory, Heidelberg, Germany.
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D000595:Amino Acid Sequence; D001483:Base Sequence; D002135:Calcium-Binding Proteins / Q000235:genetics* / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism; D002451:Cell Compartmentation; D003001:Cloning, Molecular; D017931:DNA Primers / Q000737:chemistry; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D000091342:Endoplasmic Reticulum Chaperone BiP; D005455:Fluorescent Antibody Technique; D006367:HeLa Cells; D006360:Heat-Shock Proteins; D006801:Humans; D018832:Molecular Chaperones; D008969:Molecular Sequence Data; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D014018:Tissue Distribution
D002135:Calcium-Binding Proteins; D002352:Carrier Proteins; D017931:DNA Primers; D000091342:Endoplasmic Reticulum Chaperone BiP; D006360:Heat-Shock Proteins; D018832:Molecular Chaperones; C088393:RCN2 protein, human