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 Ca²⁺-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 Ca²⁺-binding proteins that are specifically located in the ER.