Proconvertase proteolytic processing of an enzymatically active myeloperoxidase precursor

Abstract

Optimal and efficient killing of ingested microbes by human neutrophils is mediated in large part by the action of hypochlorous acid produced by the myeloperoxidase-H2O2-chloride system in phagosomes. Myeloperoxidase gene transcription is limited to early myeloid precursors in the bone marrow, when myeloperoxidase is synthesized and stored in azurophilic granules for subsequent release from stimulated neutrophils. Promyeloperoxidase, the 90-kDa myeloperoxidase precursor synthesized in the endoplasmic reticulum (ER), contains a 125-amino acid pro-region whose function and fate during myeloperoxidase biosynthesis are unknown. Promyeloperoxidase has two fates during myeloperoxidase biosynthesis; the majority undergoes proteolytic processing to generate mature myeloperoxidase, while the remainder is constituvely secreted from the cells in bone marrow. We used a promyelocytic cell line that produces endogenous myelperoxidase as well as human embryonic kidney cells stably expressing normal and mutant forms of myeloperoxidase to examine proteolytic processing of promyeloperoxidase. We demonstrated that CMK-RVKR, an inhibitor of subtilisin-like proteinases, blocked cleavage of the pro-peptide of promyeloperoxidase in a post-ER compartment. Mutants with alanine substitution of basic residues in the predicted proteinase cleavage site failed to undergo maturation to normal myeloperoxidase subunits and were arrested at the promyeloperoxidase stage. Whereas specific mutants varied as to their stability, secreted promyeloperoxidase from the mutants retained the capacity to generate hypochlorous acid. Taken together, these studies demonstrate proconvertase-dependent cleavage of promyeloperoxidase as an essential step in normal proteolytic processing and granule targeting of myeloperoxidase. Furthermore, although mutations in the proteinase cleavage site reduced intracellular stability of the mutants, the integrity of the heme group was not compromised, as chlorinating activity was retained in the secreted promyeloperoxidase.

Structure of human promyeloperoxidase (proMPO) and the role of the propeptide in processing and maturation

“…It has been proposed that the hydrolytic cleavage is mediated by a cysteine protease located in the Golgi network (21). The crystal structure of proMPO shows that the stretch 273 ASFVTG 278 was part of a surface-exposed α-helix loop segment (Fig.…”

Proprotein Convertase Processing Enhances Peroxidasin Activity to Reinforce Collagen IV

“…Besides Pxdn, PCs also cleave other animal heme peroxidases, such as MPO and TPO. Unlike Pxdn, PC processing of MPO and TPO targets the N terminus and primarily affects protein trafficking rather than catalytic activity (21,22).…”

Cellular Uptake and Delivery of Myeloperoxidase to Lysosomes Promote Lipofuscin Degradation and Lysosomal Stress in Retinal Cells

“…MPO 2 is a heme-containing peroxidase synthesized during myeloid differentiation that constitutes a major component of neutrophil azurophilic granules (1). Produced as a single chain precursor, MPO is post-translationally glycosylated in the Golgi, then cleaved into light and heavy chains in secretory azurophilic granules.…”

“…Produced as a single chain precursor, MPO is post-translationally glycosylated in the Golgi, then cleaved into light and heavy chains in secretory azurophilic granules. The mature form of MPO is a dimer composed of two light chains and two heavy chains (1). In circulating neutrophils MPO catalyzes the H 2 O 2 -mediated oxidation of halide ions to predominantly form hypochlorous acid, a strong oxidant used to kill internalized bacteria and other pathogens.…”