CALRETICULIN FUNCTIONS AS A MOLECULAR CHAPERONE IN THE BIOSYNTHESIS OF MYELOPEROXIDASE

Myeloperoxidase (MPO), a lysosomal heme protein found exclusively in n eutrophils and monocytes, is necessary for-efficient oxygen-dependent microbicidal activity. Acquisition of heme by the heme-free MPO precur sor apopro-MPO appears to be a prerequisite for its subsequent proteol ytic processing and advancement along the biosynthetic pathway to matu re MPO. We present data indicating that calreticulin (CRT), a high cap acity calcium-binding protein residing in the lumen of the endoplasmic reticulum of a wide variety of cells, interacts specifically with ful ly glycosylated apopro-MPO. Biosynthetically radiolabelled CRT (60 kDa ) and apopro-MPO (90 kDa) were coprecipitated from PLB 985 cells by mo nospecific antiserum against CRT when the immunoprecipitations were pe rformed either under nondenaturing conditions or following reversible crosslinking, Nonglycosylated MPO precursors synthesized in the presen ce of tunicamycin did not interact with CRT, The CRT-apopro-MPO intera ction was restricted to an early phase of MPO biosynthesis, and CRT di d not interact with the later appearing, heme containing species of MP O, i.e, pro-MPO or the heavy subunit of mature MPO. These data show th at CRT participates in the post-translational processing of MPO, perha ps by maintaining apopro-MPO in a conformation competent to accommodat e 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 expr essed in specialized cells.