CRISPR-associated endoribonuclease Cas13a

UniProtKB accession:  C7NBY4

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UniProtKB description:  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).