Calpain is a signal transducer and activator of transcription (STAT) 3 and STAT5 protease.

Truncation of signal transducer and activator of transcription (STAT) 5 at the carboxy-terminal domain, either by genetic engineering or by proteolytic cleavage, results in generation of dominant-negative forms. A nuclear serine protease expressed in the myeloid precursor cells is known to mediate this cleavage, but other proteases responsible for this reaction were unknown. We found that calpain, a ubiquitously expressed cysteine protease, also trims STAT5 in vivo and in vitro, within the carboxy-terminal domain. Nuclear element is not necessary for calpain-mediated STAT5 cleavage, since this process occurs in platelets. We also found that STAT3 is a substrate for calpain in vivo and in vitro, indicating that calpain-mediated cleavage is a common feature of STAT3 and STAT5. Thus, our study reveals a novel pathway for posttranslational modification of STAT3 and STAT5.