Proteolysis of the missense mutants of p67^phox, a component of NADPH oxidase, in chronic granulomatous disease

Abstract

Chronic granulomatous disease (CGD) is characterized by the failure of phagocytes to kill certain bacteria and fungi. This is caused by deficiencies in one of the components of NADPH oxidase, a superoxide-producing enzyme in phagocytic leukocytes. In some cases with missense mutations in the p67^phox gene, mRNA for p67^phox is present in normal amounts; however, p67^phox protein is missing, suggesting that the missense p67^phox protein is labile in the cytosol of phagocytic cells in CGD. In this study, we evaluated the degradation of missense mutants of p67^phox using p67^phox G78E (⁷⁸Gly → Glu) and A128Val (¹²⁸Ala → Val) as target molecules. By incubation with the cytosol factions of human peripheral blood neutrophils and promyelocytic HL-60 cells, G78E and A128V were time-dependently degraded, whereas wild-type p67^phox was not degraded. Importantly, the degradation of missense mutant G78E was completely abolished by inhibitors for serine proteases such as DFP, PMSF and soybean trypsin inhibitor, but was not affected by proteasome inhibitor MG-132 and calpain inhibitor ALLN. These observations suggest that missense mutants of p67^phox are labile and likely to be degraded in CGD by cytosolic serine protease(s) in phagocytic cells, and proteasome and calpain are unlikely to be involved in the degradation.