5wtj
From Proteopedia
Crystal structure of an endonuclease
Structural highlights
Function[C2C2_LEPSD] 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 sequences complementary to antecedent mobile elements and target invading nucleic acids. CRISPR clusters are transcribed and processed into CRISPR RNA (crRNA, optimally 28 nucleotides in this system) (PubMed:27256883). When the appropriate sequences are cloned into the CRISPR array confers immunity to ssRNA(+) enterobacteria phage MS2 (PubMed:27256883). Cleaves linear ssRNA in a crRNA-dependent fashion, preferentially at U residues; has no activity on partially dsRNA, ssDNA or dsDNA (PubMed:27256883). RNA secondary structure surrounding the target influence the cleavage site and efficiency; unlike other CRISPR-Cas effectors C2c2 cleaves outside of the crRNA binding site (PubMed:27256883). In the presence of a viable RNA target others RNAs can also be degraded in vitro and probably also in vivo, suggesting this type of CRISPR-Cas might also prevent viral spread by inducing programmed cell death or dormancy (PubMed:27256883). This system has a 3' protospacer flanking site (PFS), it does not cleave when the 3' PFS is G (PFS is equivalent to PAM, the protospacer adjacent motif) (PubMed:27256883). Mutations of its active site residues results in an RNA-programmed RNA-binding protein (PubMed:27256883).[1] Publication Abstract from PubMedC2c2, the effector of type VI CRISPR-Cas systems, has two RNase activities-one for cutting its RNA target and the other for processing the CRISPR RNA (crRNA). Here, we report the structures of Leptotrichia shahii C2c2 in its crRNA-free and crRNA-bound states. While C2c2 has a bilobed structure reminiscent of all other Class 2 effectors, it also exhibits different structural characteristics. It contains the REC lobe with a Helical-1 domain and the NUC lobe with two HEPN domains. The two RNase catalytic pockets responsible for cleaving pre-crRNA and target RNA are independently located on Helical-1 and HEPN domains, respectively. crRNA binding induces significant conformational changes that are likely to stabilize crRNA binding and facilitate target RNA recognition. These structures provide important insights into the molecular mechanism of dual RNase activities of C2c2 and establish a framework for its future engineering as a RNA editing tool. Two Distant Catalytic Sites Are Responsible for C2c2 RNase Activities.,Liu L, Li X, Wang J, Wang M, Chen P, Yin M, Li J, Sheng G, Wang Y Cell. 2017 Jan 12;168(1-2):121-134.e12. doi: 10.1016/j.cell.2016.12.031. Epub, 2017 Jan 12. PMID:28086085[2] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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