6nud | pdb_00006nud
Small conformation of ssRNA-bound CRISPR_Csm complex
Structural highlights
Function[CAS10_STRTR] CRISPR (clustered regularly interspaced short palindromic repeat) is an adaptive immune system that provides protection against mobile genetic elements (viruses, transposable elements and conjugative plasmids). CRISPR clusters contain spacers, sequences complementary to antecedent mobile elements, and target invading nucleic acids. CRISPR clusters are transcribed and processed into CRISPR RNA (crRNA). The type III-A Csm effector complex binds crRNA and acts as a crRNA-guided RNase, DNase and cyclic oligoadenylate synthase; binding of target RNA cognate to the crRNA is required for all activities. In a heterologous host this Csm effector complex restricts ssRNA phage MS2, suggesting it may target RNA viruses in vivo.[1] Csm functions as a non-specific ssDNase. Base-pairing between crRNA and target RNA to form a ternary Csm complex activates a ssDNase activity; target RNA cleavage suppresses the ssDNase, a temporal control that prevents uncontrolled DNA degradation. Viral RNA transcripts probably tether the Csm complex to the viral genome, recruiting Cas10 ssDNA activity which is able to degrade DNA in the transcription bubble, spatially controlling the DNase activity.[2] This subunit has a weak ssDNase activity that is dramatically activated by the ternary Csm effector complex (the crRNA, Cas proteins and a cognate target ssRNA). Target RNA and ssDNA are cleaved simultaneously, although RNase activity (of Csm3) is much faster. RNA cleavage by Csm3 is not required for ssDNase activity as Csm complex with inactive Csm3 still has ssDNase activity; however as the cleaved target RNA products dissociate away ssDNase activity decreases. Self-recognition, with subsequent repression of the ssDNase activity, occurs when the 5' handle of the crRNA bases pairs with the 3' flanking sequence of the target RNA (which would occur if the CRISPR locus were transcribed as an anti-pre-crRNA). This protein has low activity on dsDNA which is not stimulated by the Csm complex.[3] This subunit is a single-strand-specific deoxyribonuclease (ssDNase) which digests both linear and circular ssDNA; it has both exo- and endonuclease activity.[UniProtKB:B6YWB8] When associated with the ternary Csm effector complex (the crRNA, Cas proteins and a cognate target ssRNA) synthesizes cyclic oligoadenylates (cOA) from ATP, producing cyclic triadenylate (cA3) up to cyclic hexaadenylate (cA6), which is the active cOA. The enzyme is also able to cyclize pppA3 up to pppA6. cOAs are second messengers that induce an antiviral state important for defense against invading nucleic acids. Synthesis of cOA can occur with AMP plus ATP, 2'dATP or 3'dATP (but no other nucleotides), and requires a free 3'-OH ribose moiety.[4] [CSM2_STRTR] CRISPR (clustered regularly interspaced short palindromic repeat) is an adaptive immune system that provides protection against mobile genetic elements (viruses, transposable elements and conjugative plasmids). CRISPR clusters contain spacers, sequences complementary to antecedent mobile elements, and target invading nucleic acids. CRISPR clusters are transcribed and processed into CRISPR RNA (crRNA). The type III-A Csm effector complex binds crRNA and acts as a crRNA-guided RNase, DNase and cyclic oligoadenylate synthase; binding of target RNA cognate to the crRNA is required for all activities. In a heterologous host this Csm effector complex restricts ssRNA phage MS2, suggesting it may target RNA viruses in vivo.[5] Csm functions as a non-specific ssDNase. Base-pairing between crRNA and target RNA to form a ternary Csm complex activates a ssDNase activity; target RNA cleavage suppresses the ssDNase, a temporal control that prevents uncontrolled DNA degradation. Viral RNA transcripts probably tether the Csm complex to the viral genome, recruiting Cas10 ssDNA activity which is able to degrade DNA in the transcription bubble, spatially controlling the DNase activity.[6] This subunit may be involved in monitoring complementarity of crRNA and target RNA.[7] Publication Abstract from PubMedThe type III CRISPR-Cas (clustered regularly interspaced short palindromic repeats-CRISPR-associated genes) systems are bacterially encoded adaptive immune systems for defense against invading nucleic acids. They accomplish this task through the coordinated cleavage of invading substrates of single-stranded RNA and DNA (ssDNA and ssRNA) by the Csm (type III-A) or Cmr (type III-B) effector complexes. The ssRNA is complementarily bound to the CRISPR RNA (crRNA). However, the structural basis for the DNase and RNase activation of the Csm nucleoprotein complex is largely unknown. Here we report cryo-EM structures of the Csm-crRNA complex, with or without target ssRNA, at near-atomic resolution. Our cryo-EM maps allow us to build atomic models of the key macromolecular components, including Cas10, Csm2, Csm3, Csm4, crRNA and the invading ssRNA. Our structure resolves unambiguously the stoichiometry and tertiary structures of the Csm protein complex and the interactions between protein components and the crRNA/ssRNA. Interestingly, the new atomic structures of the Csm proteins presented here are similar to those of previously known Csm proteins in other species despite their low sequence similarity. Our combined structural and biochemical data suggest that ssRNA cleavage is preferentially carried out near its 5'-end, that the extent of interactions among the ssRNA, crRNA and the protein components regulates the DNase activity of the Csm complex, and that the 3' flanking sequence of ssRNA activates the Cas10 DNase activity allosterically. Coupling of ssRNA cleavage with DNase activity in type III-A CRISPR-Csm revealed by cryo-EM and biochemistry.,Guo M, Zhang K, Zhu Y, Pintilie GD, Guan X, Li S, Schmid MF, Ma Z, Chiu W, Huang Z Cell Res. 2019 Feb 27. pii: 10.1038/s41422-019-0151-x. doi:, 10.1038/s41422-019-0151-x. PMID:30814678[8] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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