Identification of an Evolutionarily Conserved Ankyrin Domain-Containing Protein, Caiap, Which Regulates Inflammasome-Dependent Resistance to Bacterial Infection.

Sylwia D Tyrkalska,Francisca Alcaraz-Pérez,María L Cayuela,Ana B Pérez-Oliva,Victoriano Mulero,Diana García-Moreno,Sergio Candel,Ana Valera

doi:10.3389/fimmu.2017.01375

Abstract

Many proteins contain tandemly repeated modules of several amino acids, which act as the building blocks that form the underlying architecture of a specific protein-binding interface. Among these motifs and one of the most frequently observed is ankyrin repeats (ANK), which consist of 33 amino acid residues that are highly conserved. ANK domains span a wide range of functions, including protein–protein interactions, such as the recruitment of substrate to the catalytic domain of an enzyme, or the assembly of stable multiprotein complexes. Here, we report the identification of an evolutionarily conserved protein, that we term Caiap (from CARD- and ANK-containing Inflammasome Adaptor Protein), which has an N-terminal CARD domain and 16 C-terminal ANK domains and is required for the inflammasome-dependent resistance to Salmonella Typhimurium in zebrafish. Intriguingly, Caiap is highly conserved from cartilaginous fish to marsupials but is absent in placental mammals. Mechanistically, Caiap acts downstream flagellin and interacts with catalytic active Caspa, the functional homolog of mammalian caspase-1, through its ANK domain, while its CARD domain promotes its self-oligomerization. Our results therefore point to ANK domain-containing proteins as key inflammasome adaptors required for the stabilization of active caspase-1 in functionally stable, high molecular weight complexes.

Highlights

The innate immune system detects the presence of microbes and initiates mechanisms to eliminate potentially infectious threats
A PFAM search to identify proteins harboring CARD domains revealed the presence of Caiap (CARD-ankyrin repeats (ANK) Inflammasome Adaptor Protein) in the zebrafish
The functional relevance of this extended array of NOD-like receptors (NLRs) genes in zebrafish still needs to be investigated, recent functional studies have shown that the mechanisms of activation of the inflammasome are not fully conserved in ray-finned fish and that IL-1β is not processed in vivo by caspase-1 in this animal group [16, 19, 44, 51]

Summary

Introduction

The innate immune system detects the presence of microbes and initiates mechanisms to eliminate potentially infectious threats. Microbial detection is achieved through germline-encoded pattern-recognition receptors (PRRs) that survey both the extracellular and intracellular spaces for pathogen-associated molecular patterns (PAMPs) [1]. NOD-like receptors (NLRs) are major PRRs responsible for intracellular defense, which mediate caspase-1 processing and, thereby, the activation of pro-inflammatory cytokines, such as interleukin-1β (IL-1β) and IL-18, and the induction of a special program of cell death called pyroptosis [2]. The diversity of effector domains (e.g., PYD or CARD) allows the NLRs to interact with a wide variety of binding partners, leading to the activation. Most NLRs have the ability to recruit the adaptor protein ASC, which possesses C-terminal CARD and N-terminal PYD domains. ASC has been shown to form multiprotein complexes with NLRs and caspase-1 via PYD–PYD and CARD–CARD homotypic interactions, respectively [5,6,7]

Methods

Results

Conclusion