Proconvertase proteolytic processing of an enzymatically active myeloperoxidase precursor.

Area of Science:

• Biochemistry
• Cell Biology
• Immunology

Background:

• Human neutrophils kill ingested microbes using hypochlorous acid, produced by the myeloperoxidase (MPO)-H2O2-chloride system.
• MPO is synthesized as a precursor (promyeloperoxidase) in myeloid cells and stored in granules for release upon neutrophil stimulation.

Purpose of the Study:

• To investigate the function and processing of the pro-region of promyeloperoxidase (proMPO).
• To determine the role of proteolytic cleavage in proMPO maturation and MPO targeting to neutrophil granules.

Main Methods:

• Utilized a promyelocytic cell line and human embryonic kidney cells expressing normal and mutant MPO.
• Employed a subtilisin-like proteinase inhibitor (CMK-RVKR) to study proMPO processing.
• Analyzed mutants with altered predicted proteinase cleavage sites.

Main Results:

• CMK-RVKR inhibited proMPO pro-peptide cleavage in a post-ER compartment.
• Mutants with altered cleavage sites failed to mature into normal MPO subunits and were arrested as proMPO.
• Secreted proMPO from mutants retained hypochlorous acid-generating capacity.

Conclusions:

• Proconvertase-dependent cleavage of proMPO is essential for its normal proteolytic processing and granule targeting.
• Mutations affecting cleavage sites reduce intracellular stability but not the chlorinating activity of secreted proMPO.