In vitro reconstitution of interactions in the CARD9 signalosome

The caspase-associated recruitment domain (CARD)-containing protein 9 (CARD9) signalosome is composed of CARD9, B-cell CLL/lymphoma 10 (BCL10) and mucosa-associated lymphoid tissue lymphoma translocation protein 1 (MALT1). The CARD9 signalosome has been reported to exert critical functions in the immunoreceptor tyrosine-based activation motif-coupled receptor-mediated activation of myeloid cells, through nuclear factor-κB pathways during innate immunity processes. During CARD9 signalosome assembly, BCL10 has been revealed to function as an adaptor protein and to interact with CARD9 via CARD-CARD interactions; BCL10 also interacts with MALT1 via its C-terminal Ser/Thr-rich region and the first immunoglobulin domain of MALT1. The CARD9 signalosome is implicated in critical biological processes; however, its structural and biochemical characteristics have yet to be elucidated. In the present study, CARD9 and BCL10 CARDs were successfully purified and characterized, and their biochemical properties were investigated. In addition, CARD9-BCL10 complexes were reconstituted in vitro under low salt and pH conditions. Furthermore, based on structural modeling data, a scheme was proposed to describe the interactions between CARD9 and BCL10. This provides a further understanding of the mechanism of how the CARD9 signalosome may be assembled.