CARD12: Proteins and Enzymes

Ipaf (also known as Clan/CARD12) is a CARD domain containing protein. CARD (caspase-associated recruitment domain) proteins are key regulators of cell death, cell survival and cytokine production (reviewed in Damiano and Reed, 2004). In general CARD proteins are implicated in host defense against infection, environmental stress or cellular damage. CARD domains are found in the N-terminal pro-domains of certain caspases, a family of apoptotic and pro-inflammatory proteases, as well as in a diversity of other proteins including Ipaf/Clan/CARD12. CARD domains are homotypic protein interaction motifs that enable networks of proteins to communicate via CARD-CARD interactions. There are at least three major signaling pathways in which CARD proteins act: (1) Regulation of caspase activation in the context of apoptosis (2) Regulation of caspase activation in the context of inflammation (3) Regaultion of NF-kB activation in the context of innate or adaptive immune responses. As there is significant crosstalk between pathways that lead to caspase-mediated apoptosis or inflammation and pathways that result in NF-kB activation, it is logical that similar protein modules such as CARD domains are found repeatedly in proteins from all three pathways. Ipaf plays a role in regulating caspase-1 activity, which in turn mediates the maturation of inflammatory cytokines IL-1b and IL-18 (reviewed in Lu et al, 2005). In transfected cells, Ipaf has been shown to directly interact with procaspase-1 and induce proteolytic activation of procaspase-1 in transfected cells. On the flip side, macrophages from IPAF deficient mice failed to activate caspase-1 in response to Salmonella typhimurium infection underscoring the importance of IPAF in vivo. IPAF also interact with the pro-apoptotic adaptor protein ASC and co-expression of IPAF with ASC has been shown to induce NF-kB activation and apoptosis.