Formation of intrachain disulfide bonds gives rise to two different forms of the murine IL-1 beta precursor.

Abstract

IL-1 beta is an inflammatory cytokine produced by activated macrophages. Secretion of IL-1 beta comprises a biologic inactive precursor (molecular mass = 31 kDa) and a bioactive low molecular mass peptide (17 to 20 kDa). The questions of where IL-1 beta processing takes place and what is its relationship to secretion remain to be analyzed. Here we report the novel finding that lysates of murine macrophages contain two forms, with molecular masses of 31 and 35 kDa, of the IL-1 beta precursor that can be distinguished by their different electrophoretic mobilities under nonreducing conditions. The more rapid migration of the 31-kDa polypeptide was due to a disulfide-mediated protein folding, as concluded from the following evidence. The native 31-kDa polypeptide could be unfolded to a 35-kDa polypeptide by reduction, and it spontaneously refolded to the 31-kDa polypeptide when the reducing agent was removed. Refolding of the reduced 31-kDa polypeptide was blocked in the presence of iodoacetamide, indicating that alkylation of the protein prevented the re-oxidation of disulfides. Culture supernatants contained predominantly the 31-kDa polypeptide and the mature IL-1 beta with a low m.w. of 20,000. Little or no 35-kDa IL-1 beta was detected extracellularly. These data indicate that murine macrophages contain two populations of the IL-1 beta precursor, one of which can undergo a disulfide-mediated protein folding; they also suggest that oxidation of -SH groups may be critical for the proteolytic processing of the IL-1 beta precursor.