AcrVA1 is a Moraxella bovoculi anti-CRISPR protein that binds Cas12a-crRNA complexes and inhibits type V-A CRISPR interference. It occupies the PAM-recognition region and promotes Cas12a-dependent endonucleolytic cleavage within the guide spacer. The released 3-prime spacer fragment leaves the 5-prime handle associated with Cas12a. Multiple turnover allows substoichiometric AcrVA1 to disable existing guide-loaded complexes. Inhibition has been demonstrated for several tested Cas12a orthologues; the native mobile-element context and physiological target remain less well established.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0098672 symbiont-mediated suppression of host CRISPR-cas system | IDA PMID:30936531 Broad-spectrum enzymatic inhibition of CRISPR-Cas12a. | NEW | Summary: AcrVA1 directly inhibits tested Cas12a complexes through PAM-site occlusion and guide-spacer cleavage. Reason: Purified-component experiments demonstrate that AcrVA1 performs the inhibitory step. PDB 6NMD chain B maps the structurally characterized protein to this accession. The process assignment describes the established anti-CRISPR mechanism; the native element-host pairing is not established by these heterologous assays. Supporting Evidence: PMID:30936531 AcrVA1 blocked dsDNA cleavage by all three Cas12a orthologs PMID:30936531 AcrVA1 is a multiple-turnover inhibitor that triggers cleavage of the target-recognition sequence of the Cas12a-bound guide RNA to irreversibly inactivate the Cas12a complex. |
| GO:0043021 ribonucleoprotein complex binding | IDA PMID:31155345 Structural Basis for the Inhibition of CRISPR-Cas12a by Anti... | NEW | Summary: AcrVA1 binds the assembled Cas12a-crRNA ribonucleoprotein at the PAM-recognition pocket. Reason: The cryo-EM structure directly supports binding to the guide-loaded effector; PDB 6NMD chain B maps to residues 5-174 of this accession. Supporting Evidence: PMID:31155345 AcrVA1 is sandwiched between the recognition (REC) and nuclease (NUC) lobes of Cas12a and inserts into the binding pocket for the protospacer-adjacent motif (PAM), a short DNA sequence guiding Cas12a targeting. PMID:30936531 AcrVA1 had no effect on the integrity of mature or pre-crRNA in the absence of Cas12a |
| GO:0004521 RNA endonuclease activity | IC PMID:30936531 Broad-spectrum enzymatic inhibition of CRISPR-Cas12a. | NEW | Summary: In the presence of the Cas12a-crRNA complex, AcrVA1 drives endonucleolytic cleavage of the bound guide RNA within its spacer, with scissile phosphates mapped to positions five to eight, generating an intact 3-prime fragment that the effector then releases. The reaction is multiple-turnover and proceeds at sub-stoichiometric AcrVA1 concentrations, so it is catalytic rather than stoichiometric. The independent structural study states the same conclusion. Reason: Retain the previous conservative IC proposal for Cas12a-dependent RNA endonuclease activity. Knott et al. (PMID:30936531) explicitly left the catalytic component unassigned, whereas the independent Zhang et al. study (PMID:31155345) attributes cleavage to AcrVA1 and reports an RNase active site in helix alpha2. Thus an assertion that no candidate AcrVA1 catalytic residues have been reported is outdated. Cleavage is conditional on the Cas12a-crRNA complex and does not establish autonomous RNase activity on free RNA. Knott et al. found that mutation of either known Cas12a nuclease centre (RuvC or pre-crRNA processing) did not prevent spacer truncation. Nevertheless, if AcrVA1 instead allosterically licenses another latent Cas12a RNase centre, Cas12a would perform the cleavage and AcrVA1 would be its activator; this alternative is why the molecular-function proposal remains IC rather than unqualified direct evidence of autonomous AcrVA1 catalysis. Supporting Evidence: PMID:30936531 we show that AcrVA1 triggers multiple turnover endoribonucleolytic cleavage of a Cas12a-bound crRNA to truncate the spacer sequence and permanently inactivate the complex PMID:30936531 we mapped the scissile phosphates at positions five to eight within the crRNA spacer PMID:31155345 AcrVA1 cleaves crRNA in a Cas12a-dependent manner, inactivating Cas12a-crRNA complexes. PMID:30936531 we targeted Cas12a’s nucleases for mutagenesis, the RuvC or pre-crRNA processing nuclease, and observed that mutation of either did not prevent AcrVA1-triggered spacer truncation |
| GO:0005737 cytoplasm | IC PMID:30936531 Broad-spectrum enzymatic inhibition of CRISPR-Cas12a. | NEW | Summary: Cytoplasmic action is inferred from inhibition of intracellular Cas12a-crRNA complexes. Reason: This is a bacterial UniProt record, so cytoplasm is the appropriate conservative location for the previously proposed IC annotation. A particular natural mobile element and host pair have not been established. Supporting Evidence: PMID:30936531 AcrVA1 had no effect on the integrity of mature or pre-crRNA in the absence of Cas12a PMID:31155345 AcrVA1 is sandwiched between the recognition (REC) and nuclease (NUC) lobes of Cas12a and inserts into the binding pocket for the protospacer-adjacent motif (PAM), a short DNA sequence guiding Cas12a targeting. |
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Download this section (compressed HTML)Q: Which native mobile element carries this AcrVA1 and which Cas12a system selects for its activity in Moraxella bovoculi?
Q: How do the proposed helix-alpha2 catalytic residues distinguish chemical catalysis from positioning of the crRNA substrate in the Cas12a-bound complex?
Experiment: Combine substrate-positioning structures, chemical rescue and binding-controlled mutagenesis of proposed AcrVA1 catalytic residues to distinguish direct chemical contributions from RNA positioning.
Experiment: Identify the native acrVA1 element and compare its persistence after acrVA1 deletion in matched Cas12a-positive and Cas12a-negative hosts.
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The natural host-element pairing is unestablished. Which mobile element carries acrVA1, and which Cas12a system it evolved against, are not determined, so the biological setting in which this counter-defence operates is unknown.
OPEN BIOLOGY
What is known: Both the biochemistry and the structure were done with Cas12a orthologues chosen for their divergence and their use as genome-editing reagents, including enzymes from Lachnospiraceae and Acidaminococcus, not with a demonstrated natural host-element pair.
Significance: Broad-spectrum inhibition across engineered orthologues is a reagent property. Whether AcrVA1 confers a fitness advantage on a mobile element challenged by a Cas12a system in its own ecological setting is a separate question, and it is the question that justifies the symbiont-mediated process term biologically rather than only mechanistically.
What would resolve it: Identify the element context of acrVA1 in the Moraxella bovoculi genome, then test whether deleting it costs that element persistence in a Cas12a-competent host, which is the necessity test that grounded the original anti-CRISPR assignments.
Provenance (the field's own admissions):
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