Degradation of transcription factors, c‐Jun and c‐Fos, by calpain

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Abstract
C‐Jun protein, and AP1/PEA1 transcription factor component, is a typical short‐lived protein, and like other short‐lived proteins such as c‐Fos, contains PEST regions. Calcium‐dependent neutral protease (calpain), a candidate for the degradation of PEST‐containing proteins, digests c‐Jun and c‐Fos efficiently in vitro. This is the first demonstration that transcription factors are substrates for calpain. The C‐terminal portion of c‐Jun is relatively resistant to calpain such that an 18kDa fragment, which includes the DNA binding domain, accumulates under moderate digestion conditions. The activity of c‐Jun in cultured cells can be modified by changing the level of calpastatin, an endogenous calpain inhibitor, indicating that c‐Jun is also a substrate for calpain in vivo.