Phage display epitope mapping of human neutrophil flavocytochrome b₅₅₈. Identification of two juxtaposed extracellular domains

Despite extensive experimental and clinical evidence demonstrating the critical role of flavocytochrome b₅₅₈ (Cyt b) in the NADPH-dependent oxidase, there is a paucity of direct structural data defining its topology in the phagocyte membrane. Unlike other Cyt b-specific monoclonal antibodies, 7D5 binds exclusively to an extracellular domain, and identification of its epitope should provide novel insight into the membrane topology of Cyt b. To that end, we examined biochemical features of 7D5-Cyt b binding and used the J404 phage display non-apeptide library to identify the bound epitope. 7D5 precipitated only heterodimeric gp91-p22^phox and not individual or denatured Cyt b subunits from detergent extracts of human neutrophils and promyelocytic leukemia cells (gp91-PLB). Moreover, 7D5 precipitated precursor gp65-p22^phox complexes from detergent extracts of the biosynthetically active gp91-PLB cells, demonstrating that complex carbohydrates were not required for epitope recognition. Epitope mimetics selected from the J404 phage display library by 7D5 demonstrated that ²²⁶RIVRG²³⁰ and ¹⁶⁰IKNP¹⁶³ regions of gp91^phox were both bound by 7D5. These studies reveal specific information about Cyt b membrane topology and structure, namely that gp91^phox residues ²²⁶RIVRG²³⁰ and ¹⁶⁰IKNP¹⁶³ are closely juxtaposed on extracytoplasmic domains and that predicted helices containing residues Gly¹⁶⁵-Ile¹⁹⁰ and Ser²⁰⁰-Glu²²⁵ are adjacent to each other in the membrane.