Reticulocalbin 2 (RCN2)

Reticulocalbin 2 is a calcium-binding protein located in the lumen of the ER, contains six conserved regions with similarity to a high affinity Ca(+2)-binding motif, the EF-hand. The predicted 317-amino acid RCN2 protein contains an N-terminal signal sequence, 6 copies of the EF-hand motif, and a C-terminal His-Asp-Glu-Leu (HDEL) sequence, which was originally defined as an ER retention motif in yeast proteins. Immunocytolocalization and cell fractionation studies demonstrated that RCN2 is a resident ER protein; the HDEL motif is required for its ER retention. RCN2 was detected by antibodies in all cell lines tested. It binds calcium in vitro. By Western blotting, in vitro translated RCN2 protein has a molecular mass of 55 kD, leading the authors to designate it the 'endoplasmic reticulum calcium-binding protein of 55 kD' (ERC55).