A Toll/Interleukin-1 Receptor (TIR) domain is a structural segment of about 135 to 200 amino acids found in proteins across many organisms, from bacteria to plants and animals. As a conserved component, its fundamental design has been maintained across vast evolutionary timescales, allowing it to function as a module for cellular communication.
A TIR domain’s structure consists of a central sheet of five parallel beta-strands flanked by five alpha-helices. This arrangement allows proteins to interact and transmit information within a cell. While the amino acid sequence can vary, the core three-dimensional structure is consistent, which highlights its role in cellular communication.
The Function of TIR Domains in Signaling
The primary function of a TIR domain is to act as a physical scaffold, enabling proteins to connect. This process is driven by protein-protein interactions, where a TIR domain on one protein binds to another. These interactions can be homotypic, involving identical TIR domains, or heterotypic, involving TIR domains from different proteins.
The binding of adaptor proteins to activated receptors initiates the formation of a larger assembly known as a signalosome. This complex brings multiple proteins together, allowing them to interact and activate the next components in the cascade. This mechanism ensures that a signal is efficiently propagated from the receptor to downstream molecules.
TIR Domains in Animal Innate Immunity
In animals, TIR domains are integral to the innate immune system. They are defining features of two receptor families: Toll-like receptors (TLRs) and Interleukin-1 receptors (IL-1Rs). These receptors act as sentinels on the cell surface or within internal compartments called endosomes, surveying for signs of microbial invasion or cellular stress.
TLRs recognize conserved molecular structures on microbes, known as pathogen-associated molecular patterns (PAMPs). For instance, the TLR4 receptor detects lipopolysaccharide (LPS), a component of gram-negative bacteria. When TLR4 binds to LPS, a conformational change causes its intracellular TIR domains to move closer together.
This dimerization creates an interaction surface that acts as a docking platform for adaptor proteins like MyD88 and MAL. The assembly of this receptor-adaptor complex initiates a signaling cascade that activates transcription factors. This leads to the production of inflammatory cytokines that orchestrate the immune response. IL-1Rs function similarly but are activated by cytokines like interleukin-1, which signal inflammation.
TIR Domains in Plant Defense Systems
Plants also rely on TIR domains for defense. In plants, TIR domains are found in a class of intracellular immune receptors called Nucleotide-binding Leucine-rich repeat proteins (NLRs). Those containing this domain are referred to as TIR-NLRs (TNLs). These receptors function inside the plant cell, guarding against pathogens that have breached outer defenses.
Unlike animal TLRs, plant NLRs recognize specific pathogen effector proteins, which are injected into plant cells to suppress immunity. When a TNL receptor recognizes an effector, it undergoes a conformational change that activates its TIR domain. This activation involves the receptor assembling into a multi-unit complex, a process known as oligomerization.
The activated TNL complex triggers a defense response known as effector-triggered immunity (ETI). A hallmark of ETI is the hypersensitive response, a form of programmed cell death localized to the infection site. By sacrificing infected cells, the plant creates a physical barrier that prevents the pathogen from spreading. The TNL’s activated TIR domain directly triggers this defensive cell death.
The Emerging Enzymatic Role of TIR Domains
While long understood as structural scaffolds for protein interactions, recent research has revealed that some TIR domains can also act as enzymes. This discovery has shifted the understanding of how these domains operate in plant, bacterial, and some animal pathways.
It is now established that certain activated TIR domains function as NAD+ hydrolases (NADases). These domains cleave nicotinamide adenine dinucleotide (NAD+), a molecule involved in cellular metabolism. The TIR domain breaks the bond in NAD+, yielding products like nicotinamide and variants of adenosine diphosphate ribose (ADPR).
This enzymatic activity has significant consequences. The rapid depletion of the NAD+ pool disrupts cellular energy balance, which can trigger programmed cell death. This mechanism is observed in the plant hypersensitive response and in animal axon degeneration, mediated by the protein SARM1. Additionally, the products of NAD+ cleavage can act as new signaling molecules, amplifying the immune alert.
Therapeutic Research and Applications
The involvement of TIR domains in inflammatory and immune pathways makes them a focus for therapeutic research. Because overactive signaling through receptors like TLRs and IL-1Rs can drive chronic inflammation, these pathways are implicated in autoimmune diseases like rheumatoid arthritis and inflammatory bowel disease. Developing drugs that block the protein-protein interactions between TIR domains offers a strategy to dampen these inflammatory responses.
Conversely, stimulating TIR domain signaling could enhance immune function. This approach could be used to develop more effective vaccine adjuvants, which are substances that boost the body’s immune response to a vaccine. Activating TIR-containing receptors may generate stronger immunological memory against infectious agents.
Applications also extend into agriculture. Understanding TIR domains in plant NLR proteins is helping in engineering crops with enhanced disease resistance. By modifying or introducing specific TNL genes, scientists aim to create plants that can better respond to pathogens. This could lead to more resilient crops and reduce the reliance on chemical pesticides.