6mua
From Proteopedia
Crystal structure of Csm1-Csm4 subcomplex in the type III-A CRISPR-Csm interference complex
Structural highlights
FunctionCAS10_THEON 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.[UniProtKB:A0A0A7HFE1] A single-strand deoxyribonuclease (ssDNase) which digests linear and circular ssDNA; has 5'-3' and 3'-5' exonuclease activity as well as a less efficient endonuclease activity. Has a minimal size requirement; 100 nucleotide ssDNA (nt) is more efficiently digested than 50 or 25 nt ssDNA, while 14 nt ssDNA is not cleaved at all. It has no activity on dsDNA or ssRNA.[1] ssDNase activity is stimulated in the ternary Csm effector complex; binding of cognate target RNA activates the ssDNase, as the target RNA is degraded ssDNA activity decreases.[UniProtKB:A0A0A7HFE1] When associated with the ternary Csm effector complex (the crRNA, Cas proteins and a cognate target ssRNA) synthesizes cyclic oligoadenylates (cOA) from ATP. cOAs are second messengers that stimulate the ssRNase activity of Csm6, inducing an antiviral state important for defense against invading nucleic acids.[UniProtKB:A0A0A7HFE1] Publication Abstract from PubMedType IotaIotaIota CRISPR-Cas systems provide robust immunity against foreign RNA and DNA by sequence-specific RNase and target RNA-activated sequence-nonspecific DNase and RNase activities. We report on cryo-EM structures of Thermococcus onnurineus Csm(crRNA) binary, Csm(crRNA)-target RNA and Csm(crRNA)-target RNA(anti-tag) ternary complexes in the 3.1 A range. The topological features of the crRNA 5'-repeat tag explains the 5'-ruler mechanism for defining target cleavage sites, with accessibility of positions -2 to -5 within the 5'-repeat serving as sensors for avoidance of autoimmunity. The Csm3 thumb elements introduce periodic kinks in the crRNA-target RNA duplex, facilitating cleavage of the target RNA with 6-nt periodicity. Key Glu residues within a Csm1 loop segment of Csm(crRNA) adopt a proposed autoinhibitory conformation suggestive of DNase activity regulation. These structural findings, complemented by mutational studies of key intermolecular contacts, provide insights into Csm(crRNA) complex assembly, mechanisms underlying RNA targeting and site-specific periodic cleavage, regulation of DNase cleavage activity, and autoimmunity suppression. Type III-A CRISPR-Cas Csm Complexes: Assembly, Periodic RNA Cleavage, DNase Activity Regulation, and Autoimmunity.,Jia N, Mo CY, Wang C, Eng ET, Marraffini LA, Patel DJ Mol Cell. 2018 Nov 27. pii: S1097-2765(18)30977-8. doi:, 10.1016/j.molcel.2018.11.007. PMID:30503773[2] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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