Biosynthesis, processing and sorting of neutrophil proteins: insight into neutrophil granule development

Neutrophil granulocytes are specialized phagocytic cells that carry a collection of granules for regulated secretion, each with distinct constituents. The granules can be classified as azurophil (primary), developed first, followed in time by specific (secondary) granules gelatinase granules, and secretory vesicles. Stage- and tissue-specific transcription factors govern the successive expression of genes for granule proteins to allow storage of the gene products in these organelle categories whose packaging is separated in time. Many of the granule proteins, in particular those of the heterogeneous lysosome-like azurophil granules, are subject to extensive post-translational proteolytic processing into mature proteins, most commonly as a post-sorting event. A selective aggregation of proteins destined for storage in granules, as discussed in this review, would facilitate their retention and eliminate a need for distinct sorting motifs on each granule protein. Aggregation of granule proteins, that are often cationic, would be assisted by the anionic serglycin proteoglycans present in neutrophils. The antibacterial granule proteins can serve as models for antibiotics and some of them possess a potentially useful therapeutic ability to bind and neutralize endotoxin. Because aberrant expression of transcription factors regulating the synthesis of granule proteins is often found in leukemia, the clarification of mechanisms regulating the timed expression of granule proteins will shed light on the maturation block in myeloid leukemias.