5xwy
Electron cryo-microscopy structure of LbuCas13a-crRNA binary complex
Structural highlights
FunctionCS13A_LEPBD 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 (spacer sequences) and target invading nucleic acids. Unlike many single-component effectors, this CRISPR-Cas system targets RNA (PubMed:27669025). CRISPR clusters are transcribed from pre-CRISPR RNA (crRNA) and processed into crRNA by this protein (PubMed:27669025, PubMed:28475872, PubMed:28757251). pre-crRNA processing yields a 5'-OH and probably a 2',3'-cyclic phosphate (PubMed:27669025). Also cleaves pre-crRNA from several other type VI-A CRISPR systems (PubMed:28475872). Cleaves linear target ssRNA in a crRNA-dependent fashion, preferentially before U residues (PubMed:27669025, PubMed:28475872). Cleavage of target ssRNA is about 80-fold faster than pre-crRNA processing and uses a different active site (PubMed:27669025). Binding a viable target RNA target activates this protein for non-specific RNA degradation in vitro (called collateral RNA degradation) (PubMed:27669025, PubMed:28475872, PubMed:28757251). Activation occurs with 10 fM target RNA (PubMed:28475872). crRNA maturation is not essential for activation of RNA degradation, but lack of mature crRNA (due to mutagenesis) decreases activation levels (PubMed:28475872). This system has a 3' protospacer flanking site in the target RNA (PFS), which is C and unavailable to base pair with crRNA (PFS is equivalent to PAM, the protospacer adjacent motif) (PubMed:28757251).[1] [2] [3] Publication Abstract from PubMedCas13a, a type VI-A CRISPR-Cas RNA-guided RNA ribonuclease, degrades invasive RNAs targeted by CRISPR RNA (crRNA) and has potential applications in RNA technology. To understand how Cas13a is activated to cleave RNA, we have determined the crystal structure of Leptotrichia buccalis (Lbu) Cas13a bound to crRNA and its target RNA, as well as the cryo-EM structure of the LbuCas13a-crRNA complex. The crRNA-target RNA duplex binds in a positively charged central channel of the nuclease (NUC) lobe, and Cas13a protein and crRNA undergo a significant conformational change upon target RNA binding. The guide-target RNA duplex formation triggers HEPN1 domain to move toward HEPN2 domain, activating the HEPN catalytic site of Cas13a protein, which subsequently cleaves both single-stranded target and collateral RNAs in a non-specific manner. These findings reveal how Cas13a of type VI CRISPR-Cas systems defend against RNA phages and set the stage for its development as a tool for RNA manipulation. The Molecular Architecture for RNA-Guided RNA Cleavage by Cas13a.,Liu L, Li X, Ma J, Li Z, You L, Wang J, Wang M, Zhang X, Wang Y Cell. 2017 Aug 10;170(4):714-726.e10. doi: 10.1016/j.cell.2017.06.050. Epub 2017 , Jul 27. PMID:28757251[4] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
| ||||||||||||||
