Biosynthesis, Processing, and Sorting of Human Neutrophil Granule Proteins. Focus on Cathepsin G, Proteinase 3, and Azurocidin

The neutrophil granulocyte contains a number of multifunctional proteins stored in different subsets of granules. The azurophil granules, synthesized early during differentiation, harbour leukocyte elastase, cathepsin G, proteinase 3, and azurocidin; the members of the serprocidin family. In order to study the biosynthesis, processing, and intracellular sorting of the proteins, human cDNA encoding the latter three proteins, in their preproform, was stably transfected to two hematopoietic rodent cell lines (RBL-1 and 32Dcl3). To investigate the importance of distinct protein motifs, mutations were made in the cDNAs prior to transfection. Cathepsin G lacking functional glycosylation site was normally activated and sorted to granules. Similarly, cathepsin G, lacking both functional catalytic site and amino-terminal propeptide, was stably transfected to both cell lines. However, preterm activation of catalytically active cathepsin G lead to impaired cell survival. Proteinase 3 was expressed and stored in a form that allowed specific recognition by human cANCA-sera from patients with Wegener's granulomatosis. The amino-terminal heptapropeptide of azurocidin was shown to be removed in a stepwise manner involving at least two different enzymes. All serprocidins acquired high-mannose oligosaccharides that were converted into complex forms. Finally, the biosynthetic windows of several neutrophil granule proteins, including myeloperoxidase, lysozyme, cathepsin D, and all four serprocidins, were characterized in an experimental in vitro model for differentiation of human bone marrow progenitor cells into neutrophils.