Studies on the synthesis and secretion of interleukin 1. I. A 33,000 molecular weight precursor for interleukin 1

Previous studies have suggested that murine interleukin 1 (IL 1) may be synthesized as a high m.w. precursor. Using specific antibodies against murine IL 1, we have analyzed the primary form of IL 1 synthesized by normal peritoneal macrophages and P388D1 cell line macrophages, and in vitro using poly (A)+ RNA from stimulated normal and cell line macrophages. In all cases, the labeled protein immunoprecipitated with the anti-IL 1 antibodies exhibited a m.w. of 33,000 on SDS gels. This 33,000 m.w. protein was not an aggregate of low m.w. IL 1. Addition of excess purified low m.w. IL 1 completely blocked the immunoprecipitation of the 33,000 m.w. protein. When cells were pulsed with [35S]methionine for 1 to 5 hr and then incubated in medium containing unlabeled methionine for 19 hr, labeled low m.w. IL 1 was detected in the culture fluid. If cells were pulsed with [35S]methionine to label the 33,000 m.w. protein and then incubated in the presence of a maximally effective concentration of the protein synthesis inhibitor, cycloheximide, the low m.w. IL 1 was still found in the culture fluid. Our results indicate that IL 1 is synthesized as a 33,000 m.w. precursor that is converted to the low m.w. form that is found in the culture fluid of stimulated murine macrophages.