Structural Biochemistry/NLR Sensors

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Overview
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NLR stands for nucleotide-binding domain and leucine-rich repeat containing. It is a protein that is conserved in animals as well as plants that provide pathogen-sensing systems. Mechanisms, which remain largely elusive, can activate the pathogen sensing systems directly or indirectly by molecules that are pathogen derived. NLR proteins are majorly stabilized by factors such as HSP90, a molecular chaperone, and SGT1 and RAR1, its co-chaperone, which have been revealed from studies with plants. In mammals, SGT1 and HSP90 have been found to be required for NLR proteins to function. This underscores the innate immune system regulatory mechanism’s evolutionary conservation. Insights that have been provided by by the SGT1-HSP90 complex structure from comparative analyses of mammalian and plant NLR proteins have uncovered mechanisms in the regulation of immune NLR sensors.

Plants and Animals: NLR-type Immune Sensors
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Higher eukaryotic organisms have used their intra or extracellular sensors in the initiation of disease defense responses. This was the initial switch in recognizing potential pathogens. Plants and animals both have sensors that share similar structures. NB, which is the nucleotide binding site contained in cytosolic sensors and LRR, which is the leucine rich repeat domains together are both called NLR, NB and LRR containing sensors. NLR sesnors in higher plants have recognized by directly or indirectly the specific pathogen effecter protein that promote virulence upon the delivery to the host cells. R genes, also known as resistance genes, which are genes that encode NLR proteins, have been characterized and isolated from a large variety of species of plants from the past fifteen years.

The genome of arabidopsis thatliana contains around one hundred and fifty genes that are NLR encoding. Rice has up to six hundred NLR encoding genes. Because NLR genes have the ability to identify and fight pathogens, they are very important in the breeding of agriculture. Twenty one NLR proteins in humans, which are also called caterpillar proteins or NOD-like proteins, have been involved with sensing pathogen products as well as their danger signals. They also participate in innate immune responses’ regulation.

In some humans, mutations of NLR genes are related to autoimmune diseases. Mammalian and plant NLR proteins both need proper regulation that takes a molecular chaperone HSP90 that is contained in a complex and SGT1, which can be found in bona fide co chaperone for HSP90.

There have been the emergences of three major functions concerning the large amount of data that have been amassed for HSP90. These three functions are:

  • regulating a substrates turnover by contributing to the quality control, assembly, and folding.
  • maintaining the client proteins in a metastable inactive state until a stimulus has been triggered to allow the proteins to stay as being on the verge of activation.
  • buffers the build up of cryptic mutations, which would eventually lead to unstable and inactive protein synthesis, a role involved in evolutionary processes.

There have been substantial evidence that the complex SGT1-HSP90 contributes to NLR protein’s maturation and stabilization. As of today, it is still unclear about how the process works. Dissecting the networks connecting NLR proteins, SGT1, and HSP90 should lead to the specifics of this family. This puzzle is being solved by comparative analysis of mammalian and plant regulation mechanisms as well as the structure of the core complex of SGT1-HSP90.

NLR Protein Family Characteristics
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Animals and plants have NLR proteins that harbor a central domain of NB. Throughout the three kingdoms of life, members of NLR participate in this process of complex signal transduction. The molecular recognition that is pattern specific is performed by the LRR domain that is highly variable. This LRR domain is part of the NLR subfamily. NLR proteins have N-terminal domains that bind directly to other host proteins. They have a purpose of sensing specific pathogen effectors or they participate in the recruitment of downstream signaling partners.

Not much is known so far about the details that contribute to the tight coupling in the NLR proteins of the three domains.

NLR function in Plants: Crucial Components
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HSP90, SGT1, and RAR1 are the three partner proteins that are important for many NLR sensors’ activity. These three partner proteins play an important role in resistance of plants against diseases through a network of intricate interaction. Because of these observations, it has pointed that there may be a possibility that RAR1 and SGT1 may act as a co-chaperone to HSP90. A molecular chaperone that is highly conserved is HSP90. In eukaryotic cells, they are involved in key signaling protein maturation, stabilization, and assembly. Some of these proteins include hormone receptors and protein kinases. These proteins contain an N-terminal that is use for ATP-binding domain, a middle domain that is used for binding substrate proteins, and also a C-terminal that is used for constitutive dimerization domain. The dimers work with co-chaperone proteins that regulate activity of ATPase or substrate recruitment. In a two-hybrid yeast and genetic screens, there has been shown that the HSP90 is associated with RAR1 and SGT1 through the N-terminal ATP-binding domain. It has also been shown that SGT1 and RAR1 bind to each other as well.

In eukaryotes, SGT1 is conserved. Precise functions of SGT1 are still unclear as to the details about its control on a number of processes that are unrelated. This ranges from ubiquitin ligase activation to the assembly of yeast and human kinetochore. Also included are Polo kinase and adenylyl cyclase. There are three distinct domains. They are TPR, which is tetratricopeptide repeats, CS, which is SGT1 and CHORD-containing protein, and SGS, which is SGT1 specific domain. Domains of CS and TPR are both stable and globular. The domain for SGS is unfolded intrinsically. One of the domains, the CS domain, is related evolutionarily to the co-chaperone p23. Its association to HSP90 is mediated by an interaction directly with SGT1 CS as well as HSP90 ND. On the other hand, the domain of TPR has no interactions with that of HSP90, even though some TPR domains can recognize the C-terminal pentapeptide of HSP90, which is MEEVD. SGT1 functionality was not affected by the TPR domain.

Reference
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NLR sensors meet at the SGT1-HSP90 crossroad. Kadota Y, Shirasu K, Guerois R. Trends Biochem Sci. 2010 Apr;35(4):199-207. Epub 2010 Jan 22. Review.