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Structure and ligand binding of the ADP-binding domain of the NAD+ riboswitch.

ABSTRACT: The nadA motif is the first known NAD+-dependent riboswitch, comprising two similar tandem bulged stem-loop structures. We have determined the structure of the 5' domain 1 of the riboswitch. It has three coaxial helical segments, separated by an ACANCCCC bulge and by an internal loop, with a tertiary contact between them that includes two C:G base pairs. We have determined the structure with a number of ligands related to NADH, but in each case only the ADP moiety is observed. The adenosine adopts an anti conformation, forms multiple hydrogen bonds across the width of the sugar edge of the penultimate C:G base pair of the helix preceding the bulge, and the observed contacts have been confirmed by mutagenesis and calorimetry. Two divalent metal ions play a key structural role at the narrow neck of the bulge. One makes direct bonding contacts to the diphosphate moiety, locking it into position. Thus the nucleobase, ribose, and phosphate groups of the ADP moiety are all specifically recognized by the RNA. The NAD+ riboswitch is modular. Domain 1 is an ADP binding domain that may be ancient and could potentially be used in combination with other ligand binding motifs such as CoA.

SUBMITTER: Huang L

PROVIDER: S-EPMC7297122 | biostudies-literature | 2020 Jul

REPOSITORIES: biostudies-literature

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Structure and ligand binding of the ADP-binding domain of the NAD+ riboswitch.

Huang Lin L Wang Jia J Lilley David M J DMJ

RNA (New York, N.Y.) 20200415 7

The nadA motif is the first known NAD+-dependent riboswitch, comprising two similar tandem bulged stem-loop structures. We have determined the structure of the 5' domain 1 of the riboswitch. It has three coaxial helical segments, separated by an ACANCCCC bulge and by an internal loop, with a tertiary contact between them that includes two C:G base pairs. We have determined the structure with a number of ligands related to NADH, but in each case only the ADP moiety is observed. ...[more]

PMID: 32295864

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Project description:The emergence and continued dominance of a Streptococcus pyogenes (group A Streptococcus, GAS) M1T1 clonal group is temporally correlated with acquisition of genomic sequences that confer high level expression of cotoxins streptolysin O (SLO) and NAD+-glycohydrolase (NADase). Experimental infection models have provided evidence that both toxins are important contributors to GAS virulence. SLO is a cholesterol-dependent pore-forming toxin capable of lysing virtually all types of mammalian cells. NADase, which is composed of an N-terminal translocation domain and C-terminal glycohydrolase domain, acts as an intracellular toxin that depletes host cell energy stores. NADase is dependent on SLO for internalization into epithelial cells, but its mechanism of interaction with the cell surface and details of its translocation mechanism remain unclear. In this study we found that NADase can bind oropharyngeal epithelial cells independently of SLO. This interaction is mediated by both domains of the toxin. We determined by NMR the structure of the translocation domain to be a β-sandwich with a disordered N-terminal region. The folded region of the domain has structural homology to carbohydrate binding modules. We show that excess NADase inhibits SLO-mediated hemolysis and binding to epithelial cells in vitro, suggesting NADase and SLO have shared surface receptors. This effect is abrogated by disruption of a putative carbohydrate binding site on the NADase translocation domain. Our data are consistent with a model whereby interactions of the NADase glycohydrolase domain and translocation domain with SLO and the cell surface increase avidity of NADase binding and facilitate toxin-toxin and toxin-cell surface interactions. IMPORTANCE NADase and streptolysin O (SLO) are secreted toxins important for pathogenesis of group A Streptococcus, the agent of strep throat and severe invasive infections. The two toxins interact in solution and mutually enhance cytotoxic activity. We now find that NADase is capable of binding to the surface of human cells independently of SLO. Structural analysis of the previously uncharacterized translocation domain of NADase suggests that it contains a carbohydrate binding module. The NADase translocation domain and SLO appear to recognize similar glycan structures on the cell surface, which may be one mechanism through which NADase enhances SLO pore-forming activity during infection. Our findings provide new insight into the NADase toxin and its functional interactions with SLO during streptococcal infection.

Dataset Information

Structure and ligand binding of the ADP-binding domain of the NAD+ riboswitch.

Publications

Structure and ligand binding of the ADP-binding domain of the NAD<sup>+</sup> riboswitch.

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