Citrullination as a novel posttranslational modification of matrix metalloproteinases

•We discovered citrullination as a new posttranslational modification of MMPs.•We show in vitro  that hypercitrullination of human MMP-9 alters the proteolytic activation, gelatin-binding capacity and action of an inhibitor of MMP-9.•We present ex vivo evidence of the presence of citrullinated/homocitrullinated MMP-9 in sputum samples of patients with cystic fibrosis, providing a biological link with a neutrophil-associated pathology.