3 exons spanning 1.4 kb, with a CpG island surrounding exon 1 (Conway et al., 2000; Gerhard et al., 2004; Ota et al., 2004). Exon 1 encodes a pyrin domain (PYD), exon 2 encodes a proline and glycine-rich (PGR) domain, and exon 3 encodes a caspase recruitment domain (CARD) (Masumoto et al., 1999; Matsushita et al., 2009).

Silencing of PYCARD correlates with hypermethylation of the CpG island surrounding exon 1. Breast cancer cell lines exhibit complete methylation of PYCARD and do not express PYCARD mRNA, whereas overexpression of PYCARD inhibits the growth of breast cancer cells (Conway et al., 2000).
In normal fibroblasts, the CpG island of the PYCARD gene is composed of an unmethylated domain with distinct 5-prime and 3-prime boundaries. De novo or aberrant methylation of the PYCARD CpG island in cells is accompanied by localized hypoacetylation of histone H3 and H4 and gene silencing (Stimson and Vertino, 2002).

Cytoplasm, endoplasmic reticulum, mitochondrion, nucleus.
PYCARD forms hollow spherical aggregates near the perinuclear space of apoptotic cells (McConnell and Vertino, 2000). PYCARD also tends to self-aggregate during in vitro apoptosis induced by retinoids, etoposide and other anti-tumor drugs (Masumoto et al., 1999).
PYCARD is localized primarily in the nucleus in resting monocytes/macrophages but rapidly redistributes to the cytoplasm, perinuclear space, endoplasmic reticulum and mitochondria upon pathogen infection and subsequent inflammasome activation (Bryan et al., 2009; Zhou et al., 2011).

PYCARD is known to interact with a variety of inflammatory and cell death-related genes including NLRs (NLRP1-14, NLRC4 [IL-1β converting enzyme protease-activating factor (IPAF)], Absent in Melanoma 2 (AIM2); caspase-1, caspase-2, caspase-3, caspase-5, caspase-8, caspase-9, caspase-12; pyrin; pyrin-only protein (POP) 1 and pyrin-only protein (POP) 2; cAMP-dependent protein kinase type I-alpha regulatory subunit (PRKAR1A); AP-1; serum response factor. There are 75 genes known to be induced by PYCARD. A large proportion of them are related to transcription (23%), inflammation (21%), or cell death (16%) (Hasegawa et al., 2009).

Inflammation
PYCARD is an adaptor protein involved in the structure and function of inflammasomes. Inflammasomes are pattern recognition receptors characteristically composed of an NLR, ASC and caspase-1 and are responsible for production of pro-inflammatory cytokines, in particular IL-1β and IL-18. There are several subtypes of inflammasomes that recognize a diverse array of microbial, endogenous, and environmental danger signals (Agostini et al., 2004; Mariathasan et al., 2004; Muruve et al., 2008; Fernandes-Alnemri et al., 2009; Hornung et al., 2009; Zhou et al., 2011; Dunn et al., 2012).
Mounting evidence indicates that inflammasomes and PYCARD also elicit non-overlapping inflammatory functions. PYCARD interaction with NLRC4 regulates both apoptosis via caspase-8 and NF-κB activation via PYD. PYCARD can inhibit or activate NF-κB through PYD interactions with the NF-κB IKK complex (Stehlik et al., 2002; Masumoto et al., 2003; Sarkar et al., 2006; Fernandes-Alnemri et al., 2007; Hasegawa et al., 2009; Hornung et al., 2009; Taxman et al., 2011).
PYCARD is also associated with inflammasome-independent transcriptional activation of cytokines and chemokines via activator protein-1 (AP-1), NF-κB, mitogen activated protein kinase (MAPK) and caspase-8 (Taxman et al., 2006). In pathogen-infected cells, PYCARD regulates MAPK phosphorylation by pathogens and Toll-like receptor (TLR) agonists via suppression of the dual-specificity phosphatase (DUSP10/MKP5), and independent of caspase-1 and IL-1β; thus demonstrating a function for ASC that is distinct from the inflammasome in modulating MAPK activity and chemokine expression (Taxman et al., 2011).

Adaptive immunity
PYCARD may play an inflammasome-independent role in driving dendritic cells to stimulate T-cell priming for the induction of antigen-specific cellular and humoral immunity. Dendritic cell maturation stimuli activate caspase-1 in human dendritic cells. Inhibition of PYCARD and cathepsin B markedly diminishes the capacity of mature dendritic cells to stimulate antigen-specific T cells. The defective ability of PYCARD or cathepsin B-deficient dendritic cells to stimulate T cells is independent of inflammasome-mediated processing of inflammatory cytokines or priming of dendritic cells with pre-processed lipopolysaccharide (Guo and Dhodapkar, 2012).
On the other hand, PYCARD may also play an inflammasome-independent role in antigen-specific inflammatory disease. Mice genetically modified to lack both PYCARD alleles [ASC (-/-)] are protected from collagen-induced arthritis, whereas mice lacking Nlrp3 and caspase-1 are susceptible to collagen-induced arthritis. This may result from an inability of dendritic cells to facilitate antigen-specific activation of lymphocytes in mice lacking PYCARD. Furthermore, antigen-induced proliferation of purified T cells lacking PYCARD [ASC (-/-)] is restored upon incubation with wild type dendritic cells, but not when cultured with ASC (-/-) dendritic cells (Ippagunta et al., 2010).

Cell death (apoptosis, pyroptosis, necrosis)
PYCARD promotes caspase-mediated inhibition of cellular proliferation, DNA fragmentation and apoptosis via caspases including caspase-2/3/8 and 9 to activate the mitochondrial apoptotic pathway. The mechanism likely involves mitochondrial translocation of BAX, proteolytic maturation of BID and upregulation of the p53 response to cell stress or genotoxic insult (McConnell and Vertino, 2000; Ohtsuka et al., 2004; Hasegawa et al., 2007). PYCARD may also increase the susceptibility of leukemia cell lines to apoptotic stimuli by anticancer drugs (Masumoto et al., 1999).
PYCARD is involved in macrophage pyropoptosis (inflammatory cell death) which is characterized by potassium efflux and/or decreased intracellular potassium. The interaction of AIM2 with PYCARD leads to the formation of the pyroptosome, which induces pyroptotic cell death in response to cytoplasmic DNA in cells containing caspase-1 (Fernandes-Alnemri et al., 2007; Fernandes-Alnemri et al., 2009).
PYCARD also mediates cellular necrosis (pyronecrosis) in concert with NLRP3 and cathespin to cause programmed necrotic cell death that is independent from pyroptosis and does not require caspase-1 (Willingham et al., 2007; Satoh et al., 2013).

There are 4 transcripts (splice variants) including the canonical PYCARD (PYCARD1) (Matsushita et al., 2009; Bryan et al., 2010). Correlating to four transcript splice variants are four protein isoforms. In addition to the canonical PYCARD protein (also known as isoform 1, fASC), three additional isoforms display unique capabilities with respect to their function as part of the inflammasome, with one of the isoforms even showing an inhibitory effect. Isoforms 1 and 2 are the activating isoforms of ASC and co-localize with intracellular nucleotide oligomerization domain-like receptors (NLRs) and caspase-1. Isoform 2 (also known as ASC-b, vASC) lacks a PGR domain and may not be needed for caspase activation but is involved in direct regulation of IL-1β processing. The inhibitory isoform (isoform 3, ASC-c) co-localizes only with caspase-1, but not with NLRP3. Isoform 4 (ASC-d) does not co-localize with NLRP3 or with caspase-1 and lacks the ability to function as an inflammasome adaptor. It may not be a functional protein product and its precise function and relation to PYCARD is unknown (Matsushita et al., 2009; Bryan et al., 2010).
PYD is also known as the domain in apoptosis and interferon response (DAPIN) or the pyrin, AIM, ASC death-domain-like (PAAD) domain. It is an 80-100 residue domain with alpha-helical secondary structure located on the N-terminus of the protein. Like CARD, it is a member of the death domain-fold superfamily of proteins. Strong dipole moments in PYD suggest that electrostatic interactions play an important role for the binding between PYDs. The function of PYD is to bind other PYD-containing proteins and is also associated with domains such as CARD, leucine-rich repeat (LRR), dual specificity spore lysis A (splA) protein kinase and ryanodine receptor (SPRY), caspase, or zinc-finger B-box (Martinon et al., 2001; Pawlowski et al., 2001; Liepinsh et al., 2003).
CARD is a subclass of protein motif known as the death fold, which features an arrangement of six to seven antiparallel alpha helices with a hydrophobic core and an outer face composed of charged residues. The CARD structure of PYCARD reveals two distinctive characteristics; helix 1 is not fragmented as in all other known CARDs; and it demonstrates a uniform distribution of positive and negative charges, whereas these are commonly separated into two areas in other death domains (de Alba, 2009).
CARD mediates the interaction between adaptor proteins participating in apoptosis by regulating caspases. CARD-containing proteins are also involved in inflammation through their regulation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). The mechanisms by which CARDs activate caspases and NF-κB involve the assembly of multi-protein complexes, which can facilitate dimerization or serve as scaffolds on which proteases and kinases are assembled and activated. Domains associated with CARD include: PYD, Apoptotic protease activating factor-1 (Apaf-1) domains [including LRR, Tryptophan-Aspartic acid (WD or beta-transducin) repeats, nucleotide binding and oligomerization (NB-ARC or NOD) domains and ATPase domains], sarcoma (Src) tyrosine kinase proto-oncogene homology domains, death domain (DD) and the proform of caspases (e.g., CASP-9) (Hofmann et al., 1997; Bouchier-Hayes and Martin, 2002; Reed et al., 2004).
The PGR linker adopts a residual structure in order to maintain a back-to-back orientation of the PYD and CARD domains, which avoids steric interference of one domain with the binding site of the other. NMR relaxation experiments show that the linker is flexible despite the residual structure (de Alba, 2009).