Research article

The PAAD/PYRIN-only protein POP1/ASC2 is a modulator of ASC-mediated nuclear-factor-kappaB and pro-caspase-1 regulation

Christian STEHLIK, Maryla KRAJEWSKA, Kate WELSH, Stanislaw KRAJEWSKI, Adam GODZIK, John C. REED


Proteins containing PAAD {pyrin, AIM (absent-in-melanoma), ASC [apoptosis-associated speck-like protein containing a CARD (caspase-recruitment domain)] and DD (death domain)-like} (PYRIN, DAPIN) domains are involved in innate immunity, regulating pathways leading to nuclear-factor-κB (NF-κB) and pro-caspase-1 activation. Many PAAD-family proteins have structures reminiscent of Nod-1, a putative intracellular sensor of lipopolysaccharide. Hereditary mutations in some of the PAAD-family genes are associated with auto-inflammatory diseases. Several of these proteins utilize the bipartite PAAD- and CARD-containing adapter protein ASC/TMS-1 (target of methylation-induced silencing) for linking to downstream signalling pathways. In the present paper, we describe characterization of human PAAD-only protein-1 (POP1)/ASC2, which is highly homologous with the PAAD domain of ASC, and which probably originated by gene duplication on chromosome 16. We demonstrate that POP1/ASC2 associates with ASC via PAAD–PAAD interactions and modulates NF-κB and pro-caspase-1 regulation by this adapter protein. In gene transfer experiments, POP1/ASC2 suppressed cytokine-mediated NF-κB activation similar to other PAAD-family proteins previously tested. Immunohistochemical studies showed expression of POP1/ASC2 predominantly in macrophages and granulocytes. We propose that POP1/ASC2 functions as a modulator of multidomain PAAD-containing proteins involved in NF-κB and pro-caspase-1 activation and innate immunity.

  • cryopyrin
  • nuclear factor (NF-)κB inhibitor kinase complex (IKK complex)
  • innate immunity
  • interleukin-1β
  • pyrin


  • Abbreviations used: CARD, caspase-recruitment domain; ASC, apoptosis-associated speck-like protein containing a CARD; DD, death domain; DED, death-effector domain; DEFCAP, death effector filament-forming Ced-4-like apoptosis protein; DMEM, Dulbecco's modified Eagle's medium; DTT, dithiothreitol; ECL®, enhanced chemiluminescence; EST, expressed sequence tag; FCS, fetal calf serum; FLIP, Fas-associated DD-like IL-1β-converting enzyme (‘FLICE’)-inhibitory protein; GFP, green fluorescent protein; GST, glutathione S-transferase; HA, haemagglutinin; ICE, IL-1β-converting enzyme; IκB, inhibitory κB; IKK, IκB kinase; IL-1β, interleukin-1β; LPS, lipopolysaccharide; NAC, nucleotide-binding domain and CARD-containing protein; NACHT, NAIP (neuronal apoptosis inhibitory protein), CIITA (MHC class II transcription activator), HET-E (incompatibility locus protein from Podospora anserina) and TP1 (telomerase-associated protein); NALP, NACHT, LRR (leucine-rich repeat) and PYRIN protein; Neo, neomycin; NF-κB, nuclear factor κB; NP40, Nonidet P40; PAAD, pyrin, AIM (absent-in-melanoma), ASC and DD-like; PAN2, PAAD- and NACHT-containing protein; POP1, PAAD-only protein-1; PYPAF, PYRIN-containing Apaf1-like protein; RFP, red fluorescent protein; RT, reverse transcriptase; TBK1, TRAF-associated NF-κB activator (TANK)-binding kinase 1; TNF, tumour necrosis factor; TRAF, TNF-receptor-associated factor.