Multiple zones in the sequence of calreticulin (CRP55, calregulin, HACBP), a major calcium binding ER/SR protein

The complete amino acid sequence of CRP55, the major 55 kd calcium binding protein of the ER lumen, was deduced from the murine cDNA nucleotide sequence. This was completed using a novel application of PCR amplification. The mature 399 residue protein encoded is preceded by a 17 amino acid leader sequence and ends in the ER signal sequence, KDEL. The protein contains no calcium binding motifs of the EF hand type or of the form seen in calelectrin-related proteins. The major region of potential low affinity calcium binding sites is a polyacidic stretch towards the C terminus. The primary structure of the protein is markedly zonal. The N-terminal region, of approximately neutral net charge and hydrophobicity, is followed by a central proline-rich zone with repeat sequences separated from the polyacidic C-terminal stretch by a short hydrophobic sequence. The general shape suggested is a globular domain attached to an extended tail. Immunofluorescence studies show that the protein is present in skeletal muscle and indicate that it is a sarcoplasmic reticulum protein. We propose that the protein be named calreticulin to reflect its calcium binding activity and location in the ER and SR.