Different protein turnover of interleukin‐6‐type cytokine signalling components

Interleukin (IL)‐6 and IL‐6‐type cytokines signal through the gp130/Jak/STAT signal transduction pathway. The key components involved are the signal transducing receptor subunit gp130, the Janus kinases Jak1, Jak2 and Tyk2, STAT1 and STAT3 of the family of signal transducers and activators of transcription, the protein tyrosine phosphatase SHP2 and the suppressors of cytokine signalling SOCS1, SOCS2 and SOCS3. Whereas considerable information has been accumulated concerning the time‐course of activation for the individual signalling molecules, data on the availability of the proteins involved in IL‐6‐type cytokine signal transduction are scarce. Nevertheless, availability of these molecules, determined by the balance of protein synthesis and degradation, also influences IL‐6‐type cytokine signal transduction. Here, we present a comprehensive set of data on the half‐lives of the key molecules involved in the IL‐6 signal transduction pathway. The turnover rates for the various proteins differ substantially. Three groups of signalling proteins can be discriminated: whereas the feedback inhibitors SOCS1, SOCS2 and SOCS3 are very short‐lived, STAT1, STAT3 and SHP2 have an extremely slow turnover rate. Interestingly, the half‐life of STAT3β, a splice variant of STAT3α, is reduced to almost 50% of the half‐life of STAT3α. The Janus kinases Jak1, Jak2, Tyk2 and gp130 show intermediate half‐lives. Our data imply that signalling components activated by post‐translational modifications are long‐lived whereas the activity of very short‐lived proteins is regulated mainly at the transcriptional level.