Substrate-induced condensation activates plant TIR domain proteins.

Plant nucleotide-binding leucine-rich repeat (NLR) immune receptors with an N-terminal Toll/interleukin-1 receptor (TIR) domain mediate recognition of strain-specific pathogen effectors, typically via their C-terminal ligand-sensing domains1. Effector binding enables TIR-encoded enzymatic activities that are required for TIR-NLR (TNL)-mediated immunity2,3. Many truncated TNL proteins lack effector-sensing domains but retain similar enzymatic and immune activities4,5. The mechanism underlying the activation of these TIR domain proteins remain unclear. Here we show that binding of the TIR substrates NAD+ and ATP induces phase separation of TIR domain proteins in vitro. A similar condensation occurs with a TIR domain protein expressed via its native promoter in response to pathogen inoculation in planta. The formation of TIR condensates is mediated by conserved self-association interfaces and a predicted intrinsically disordered loop region of TIRs. Mutations that disrupt TIR condensates impair the cell death activity of TIR domain proteins. Our data reveal phase separation as a mechanism for the activation of TIR domain proteins and provide insight into substrate-induced autonomous activation of TIR signalling to confer plant immunity.

Structural polymorphisms within a common powdery mildew effector scaffold as a driver of coevolution with cereal immune receptors.

In plants, host-pathogen coevolution often manifests in reciprocal, adaptive genetic changes through variations in host nucleotide-binding leucine-rich repeat immune receptors (NLRs) and virulence-promoting pathogen effectors. In grass powdery mildew (PM) fungi, an extreme expansion of a RNase-like effector family, termed RALPH, dominates the effector repertoire, with some members recognized as avirulence (AVR) effectors by cereal NLR receptors. We report the structures of the sequence-unrelated barley PM effectors AVRA6, AVRA7, and allelic AVRA10/AVRA22 variants, which are detected by highly sequence-related barley NLRs MLA6, MLA7, MLA10, and MLA22 and of wheat PM AVRPM2 detected by the unrelated wheat NLR PM2. The AVR effectors adopt a common scaffold, which is shared with the RNase T1/F1 family. We found striking variations in the number, position, and length of individual structural elements between RALPH AVRs, which is associated with a differentiation of RALPH effector subfamilies. We show that all RALPH AVRs tested have lost nuclease and synthetase activities of the RNase T1/F1 family and lack significant binding to RNA, implying that their virulence activities are associated with neo-functionalization events. Structure-guided mutagenesis identified six AVRA6 residues that are sufficient to turn a sequence-diverged member of the same RALPH subfamily into an effector specifically detected by MLA6. Similar structure-guided information for AVRA10 and AVRA22 indicates that MLA receptors detect largely distinct effector surface patches. Thus, coupling of sequence and structural polymorphisms within the RALPH scaffold of PMs facilitated escape from NLR recognition and potential acquisition of diverse virulence functions.

Elucidation of tropane alkaloid biosynthesis in Erythroxylum coca using a microbial pathway discovery platform.

Tropane alkaloids (TAs) are heterocyclic nitrogenous metabolites found across seven orders of angiosperms, including Malpighiales (Erythroxylaceae) and Solanales (Solanaceae). Despite the well-established euphorigenic properties of Erythroxylaceae TAs like cocaine, their biosynthetic pathway remains incomplete. Using yeast as a screening platform, we identified and characterized the missing steps of TA biosynthesis in Erythroxylum coca. We first characterize putative E. coca polyamine synthase- and amine oxidase-like enzymes in vitro, in yeast, and in planta to show that the first tropane ring closure in Erythroxylaceae occurs via bifunctional spermidine synthase/N-methyltransferases and both flavin- and copper-dependent amine oxidases. We next identify a SABATH family methyltransferase responsible for the 2-carbomethoxy moiety characteristic of Erythroxylaceae TAs and demonstrate that its coexpression with methylecgonone reductase in yeast engineered to express the Solanaceae TA pathway enables the production of a hybrid TA with structural features of both lineages. Finally, we use clustering analysis of Erythroxylum transcriptome datasets to discover a cytochrome P450 of the CYP81A family responsible for the second tropane ring closure in Erythroxylaceae, and demonstrate the function of the core coca TA pathway in vivo via reconstruction and de novo biosynthesis of methylecgonine in yeast. Collectively, our results provide strong evidence that TA biosynthesis in Erythroxylaceae and Solanaceae is polyphyletic and that independent recruitment of unique biosynthetic mechanisms and enzyme classes occurred at nearly every step in the evolution of this pathway.

TIR-catalyzed ADP-ribosylation reactions produce signaling molecules for plant immunity.

Plant pathogen-activated immune signaling by nucleotide-binding leucine-rich repeat (NLR) receptors with an N-terminal Toll/interleukin-1 receptor (TIR) domain converges on Enhanced Disease Susceptibility 1 (EDS1) and its direct partners, Phytoalexin Deficient 4 (PAD4) or Senescence-Associated Gene 101 (SAG101). TIR-encoded nicotinamide adenine dinucleotide hydrolase (NADase) produces signaling molecules to promote exclusive EDS1-PAD4 and EDS1-SAG101 interactions with helper NLR subclasses. In this work, we show that TIR-containing proteins catalyze adenosine diphosphate (ADP)-ribosylation of adenosine triphosphate (ATP) and ADP ribose (ADPR) through ADPR polymerase-like and NADase activity, forming ADP-ribosylated ATP (ADPr-ATP) and ADPr-ADPR (di-ADPR), respectively. Specific binding of ADPr-ATP or di-ADPR allosterically promotes EDS1-SAG101 interaction with helper NLR N requirement gene 1A (NRG1A) in vitro and in planta. Our data reveal an enzymatic activity of TIRs that enables specific activation of the EDS1-SAG101-NRG1 immunity branch.

Identification and receptor mechanism of TIR-catalyzed small molecules in plant immunity.

Plant nucleotide-binding leucine-rich repeat-containing (NLR) receptors with an N-terminal Toll/interleukin-1 receptor (TIR) domain sense pathogen effectors to enable TIR-encoded nicotinamide adenine dinucleotide hydrolase (NADase) activity for immune signaling. TIR-NLR signaling requires the helper NLRs N requirement gene 1 (NRG1), Activated Disease Resistance 1 (ADR1), and Enhanced Disease Susceptibility 1 (EDS1), which forms a heterodimer with each of its paralogs Phytoalexin Deficient 4 (PAD4) and Senescence-Associated Gene 101 (SAG101). Here, we show that TIR-containing proteins catalyze the production of 2'-(5''-phosphoribosyl)-5'-adenosine monophosphate (pRib-AMP) and diphosphate (pRib-ADP) in vitro and in planta. Biochemical and structural data demonstrate that EDS1-PAD4 is a receptor complex for pRib-AMP and pRib-ADP, which allosterically promote EDS1-PAD4 interaction with ADR1-L1 but not NRG1A. Our study identifies TIR-catalyzed pRib-AMP and pRib-ADP as a missing link in TIR signaling through EDS1-PAD4 and as likely second messengers for plant immunity.

TIR domains of plant immune receptors are 2',3'-cAMP/cGMP synthetases mediating cell death.

2',3'-cAMP is a positional isomer of the well-established second messenger 3',5'-cAMP, but little is known about the biology of this noncanonical cyclic nucleotide monophosphate (cNMP). Toll/interleukin-1 receptor (TIR) domains of nucleotide-binding leucine-rich repeat (NLR) immune receptors have the NADase function necessary but insufficient to activate plant immune responses. Here, we show that plant TIR proteins, besides being NADases, act as 2',3'-cAMP/cGMP synthetases by hydrolyzing RNA/DNA. Structural data show that a TIR domain adopts distinct oligomers with mutually exclusive NADase and synthetase activity. Mutations specifically disrupting the synthetase activity abrogate TIR-mediated cell death in Nicotiana benthamiana (Nb), supporting an important role for these cNMPs in TIR signaling. Furthermore, the Arabidopsis negative regulator of TIR-NLR signaling, NUDT7, displays 2',3'-cAMP/cGMP but not 3',5'-cAMP/cGMP phosphodiesterase activity and suppresses cell death activity of TIRs in Nb. Our study identifies a family of 2',3'-cAMP/cGMP synthetases and establishes a critical role for them in plant immune responses.

Backbone and nearly complete side-chain chemical shift assignments of the human death-associated protein 1 (DAP1).

Death-associated protein 1 (DAP1) is a proline-rich cytoplasmatic protein highly conserved in most eukaryotes. It has been reported to be involved in controlling cell growth and migration, autophagy and apoptosis. The presence of human DAP1 is associated to a favourable prognosis in different types of cancer. Here we describe the almost complete [Formula: see text], [Formula: see text], and [Formula: see text] chemical shift assignments of the human DAP1. The limited spectral dispersion, mainly in the [Formula: see text] region, and the lack of defined secondary structure elements, predicted based on chemical shifts, identifies human DAP1 as an intrinsically disordered protein (IDP). This work lays the foundation for further structural investigations, dynamic studies, mapping of potential interaction partners or drug screening and development.

1H, 13C, and 15N Backbone assignments of the human brain and acute leukemia cytoplasmic (BAALC) protein.

The brain and acute leukemia cytoplasmic (BAALC; UniProt entry Q8WXS3) is a 180-residue-long human protein having six known isoforms. BAALC is expressed in either hematopoietic or neuroectodermal cells and its specific function is still to be revealed. However, as a presumably membrane-anchored protein at the cytoplasmic side it is speculated that BAALC exerts its function at the postsynaptic densities of certain neurons and might play a role in developing cytogenetically normal acute myeloid leukemia (CN-AML) when it is highly overexpressed by myeloid or lymphoid progenitor cells. In order to better understand the physiological role of BAALC and to provide the basis for a further molecular characterization of BAALC, we report here the 1H, 13C, and 15N resonance assignments for the backbone nuclei of its longest hematopoietic isoform (isoform 1). In addition, we present a 1HN and 15NH chemical shift comparison of BAALC with its shortest, neuroectodermal isoform (isoform 6) which shows only minor changes in the 1H and 15N chemical shifts.