MALT1 (Mucosa associated lymphoid tissue protein 1) is an important adapter protein for the NF-κB driven lymphocyte activation and the development and survival of distinct B-cell lymphoma entities. In addition MALT1 is a cysteine protease that structurally resembles caspases while having a different substrate preference and mechanism of activation. This paracaspase activity of MALT1 has been shown to be critical for an optimal NF-κB activation and survival of the aggressive ABC-DLBCL (activated

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... B cell-type of diffuse large B cell lymphoma), which highlights the protease as an attractive therapeutic target for the treatment of distinct B-cell lymphomas and immune diseases like rheumatoid arthritis or multiple sclerosis. In this protocol we describe a fluorogenic cleavage assay, which can be used to measure endogenous and also ectopic MALT1 activity. To this end, cellular MALT1 needs to be precipitated from the lysed cells via antibody immunoprecipitation and subsequently incubated with a fluorogenic substrate peptide. The MALT1 cleavage assay has been developed to directly determine the activity profile of MALT1 in the course of the adaptive immune response as well as in pathological signaling in lymphoid malignancies. In addition, the MALT1 activity assay has been successfully used to monitor cellular MALT1 inhibition with small molecule inhibitors. Materials and Reagents http://www.bio-protocol.org/e821