Calreticulin Functions as a Molecular Chaperone in the Biosynthesis of Myeloperoxidase (∗)

• Published in: The Journal of biological chemistry 1995-03, Vol.270 (9), p.4741-4747
• Main Authors: Nauseef, William M., McCormick, Sally J., Clark, Robert A.
• Format: Article
• Language: English
• Subjects: Calcium-Binding Proteins - metabolism; Calreticulin; Chaperonins - metabolism; Electrophoresis, Polyacrylamide Gel; Endoplasmic Reticulum - metabolism; Enzyme Precursors - metabolism; Humans; Peroxidase - biosynthesis; Protein Processing, Post-Translational; Proteins - metabolism; Ribonucleoproteins - metabolism; Tumor Cells, Cultured
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.270.9.4741

Myeloperoxidase (MPO), a lysosomal heme protein found exclusively in neutrophils and monocytes, is necessary for efficient oxygen-dependent microbicidal activity. Acquisition of heme by the heme-free MPO precursor apopro-MPO appears to be a prerequisite for its subsequent proteolytic processing and advancement along the biosynthetic pathway to mature MPO. We present data indicating that calreticulin (CRT), a high capacity calcium-binding protein residing in the lumen of the endoplasmic reticulum of a wide variety of cells, interacts specifically with fully glycosylated apopro-MPO. Biosynthetically radiolabeled CRT (60 kDa) and apopro-MPO (90 kDa) were coprecipitated from PLB 985 cells by monospecific antiserum against CRT when the immunoprecipitations were performed either under nondenaturing conditions or following reversible cross-linking. Nonglycosylated MPO precursors synthesized in the presence of tunicamycin did not interact with CRT. The CRT-apopro-MPO interaction was restricted to an early phase of MPO biosynthesis, and CRT did not interact with the later appearing, heme-containing species of MPO, i.e. pro-MPO or the heavy subunit of mature MPO. These data show that CRT participates in the posttranslational processing of MPO, perhaps by maintaining apopro-MPO in a conformation competent to accommodate insertion of the heme group. In this general way, CRT shares certain functional properties with the structurally homologous transmembrane calcium-binding endoplasmic reticulum protein calnexin. Both interact with glycosylated biosynthetic precursors of proteins selectively expressed in specialized cells.