The murine epsilon receptor modulating protein: a novel serine protease which modulates CD23 binding of IgE

In our recent previous studies, we have identified and purified a murine 17-kDa protein which diminishes the avidity of binding between IgE and CD23 (low-affinity Fc receptor for IgE) without decreasing the quantitative expression of the CD23. The protein was thus designated epsilon receptor modulating protein (epsilon RMP). In this study, we have further characterized this protein and have found that (i) epsilon RMP is inactivated by phenylmethylsulfonyl fluoride and decomposes N,alpha-benzyloxycarbonyl-L-lysine thiobenzyl ester, as well as N-succinyl-Ala-Ala-Pro-Phe p-nitroanilide; (ii) epsilon RMP does not work directly on B cells but requires CD4+ T cells to decrease functional expression of CD23 on B cells; and (iii) the partial internal amino acid sequence of epsilon RMP, obtained by using in situ cyanogen bromide cleavage on polyvinylidene difluoride membrane is unique. These data thus clearly demonstrate that epsilon RMP is a novel serine protease controlling the functional expression of CD23 through the participation of CD4+ T cells. Mechanisms of the involvement of CD4+ T cells are discussed.