Monoclonal antibody CL5 recognizes the amino terminal domain of human phagocyte flavocytochrome b₅₅₈ large subunit, gp91^phox

Human phagocyte flavocytochrome b₅₅₈ (Cytb) is a heterodimeric integral membrane protein that serves as the electron transferase of the β-nicotinamide adenine dinucleotidephosphate, reduced (NADPH)-oxidase, an enzyme complex important in the host defense function of phagocytic cells. In this study, we report the characterization of monoclonal antibody (mAb) CL5 that is specific for the large subunit, gp91^phox, of the oxidase protein. This antibody recognizes gp91^phox by immunoblot analysis of membrane extracts and samples of the immunopurified gp91^phox/p22 ^phox heterodimer, prepared on anti-p22^phox affinity matrices. Phage display analysis confirmed this specificity, indicating that the CL5 epitope contains the region 135-DPYSVALSELGDR of gp91^phox. The antibody was used to probe for the presence of gp91^phox in membrane preparations from neutrophils of patients with nine genetically distinct forms of X-linked chronic granulomatous disease (CGD). The causative mutations included missense errors as well as nonsense errors that result in premature termination of gp91^phox synthesis. Analysis of the CGD samples by immunoblotting indicated that CL5 recognizes only the full-length wild-type and two missense mutations, consistent with the absence of stable short gp91 ^phox peptide expression in CGD neutrophils. Interestingly, CL5 was also shown to be cross-reactive with cytosolic and membrane-bound gelsolin, identified by purification, mass spectrometry and immunoblot analysis. CL5 probably cross-reacts with the sequence 771-DPLDRAMAEL in the C-terminus of gelsolin. We conclude that mAb CL5 is a useful probe for detection of full length and possibly truncated N-terminal fragments of gp91^phox from membranes of Cytb-producing cells.