Cloning of a human homologue of mouse reticulocalbin reveals conservation of structural domains in the novel endoplasmic reticulum resident Ca(2+)-binding protein with multiple EF-hand motifs.

Abstract

Recently, we described the isolation of a mouse cDNA clone encoding a novel Ca(2+)-binding protein, tentatively designated as reticulocalbin [Ozawa, M. and Muramatsu, T. (1993) J. Biol. Chem. 268, 699-705]. Reticulocalbin is a lumenal protein of the endoplasmic reticulum (ER) with a molecular weight of 44,000 and has six repeats of a domain containing the high affinity EF-hand Ca(2+)-binding motif. The protein has the sequence, His-Asp-Glu-Leu (HDEL), at its carboxy terminus, which serves as a signal for its retention in the ER of cells. To examine the importance of the putative Ca(2+)-binding domains as well as the carboxy-terminal HDEL sequence, we have cloned the human homologue of reticulocalbin. The sequence of this clone revealed a novel protein with 95% identity in amino acid sequence to the mouse reticulocalbin, indicating that this molecule has been evolutionarily conserved in mammals. As was found for the mouse reticulocalbin, the human homologue showed six repeats of a domain containing EF-hand motifs. Interestingly, conservation of the amino acid sequence was not restricted to the Ca(2+)-binding motifs, consistent with the possibility that reticulocalbin plays some role(s) besides Ca(2+)-binding. As was found for the mouse homologue, the protein has the HDEL sequence at its carboxy terminus instead of the Lys-Asp-Glu-Leu (KDEL) sequence, which is more common as a signal for the retention of resident proteins in the ER of animal cells. The conservation of the HDEL sequence in reticulocalbin in both species raises the possibility that this sequence has some roles in the function(s) of this protein family.