Anti-CRISPR Proteins Project

Bottom line: anti-CRISPR (Acr) proteins are small phage proteins that switch
off bacterial CRISPR-Cas immunity, and their GO annotations lag far behind a
detailed structural literature. We reviewed two genes from Pectobacterium phage
ZF40: the Type I-F inhibitor AcrF8 and its Aca2 repressor (reviewed as
genes/BPZF4/ACA2, which the pending list below predates). For AcrF8 the
generic IEA term GO:0052170 (symbiont-mediated suppression of host innate immune response) was
modified to GO:0098672 (symbiont-mediated suppression of host CRISPR-cas system), and
GO:0043021 ribonucleoprotein complex binding was added as its core function
from the cryo-EM structure (PMID:32170016). We also proposed a new term,
"CRISPR RNA binding anti-CRISPR activity", because AcrF8 contacts the crRNA as
well as the Cas7f backbone and no current term captures that. The wider Acr
families (AcrIF, AcrIE, AcrIIA) have not been started, so the project is a
worked example rather than a family-wide review.

Overview

This project tracks the annotation review of anti-CRISPR (Acr) proteins - phage-encoded inhibitors of bacterial CRISPR-Cas immune systems. These proteins represent a fascinating example of evolutionary arms race between bacteria and their viral predators, and present unique challenges for GO annotation:

1. Novel molecular mechanisms - Acr proteins often have unique mechanisms not well-captured by existing GO terms
2. Dual-targeting strategies - Some Acrs interact with both protein and RNA components
3. Regulatory complexity - Expression is tightly regulated via Aca (anti-CRISPR associated) repressors
4. Sparse annotations - Many Acr proteins have only generic IEA annotations

These proteins are excellent targets for AI-assisted curation because their mechanisms are well-studied structurally but annotations lag behind.

Source: Presented at Gene Ontology Consortium Meeting, October 2025, Cambridge UK. See ai4curation/ai-gene-review.

Key Concepts

Anti-CRISPR Mechanisms

• Type I-F inhibitors (AcrIF family) - Target Csy surveillance complex
• Type I-E inhibitors (AcrIE family) - Target Cascade complex
• Type II inhibitors (AcrIIA/IIC) - Target Cas9/Cas12
• Type III inhibitors - Target Csm/Cmr complexes

Inhibition Strategies

• Blocking DNA recognition
• Preventing R-loop formation
• Mimicking DNA substrates
• Direct crRNA binding (unique mechanism)
• Enzymatic inactivation

Featured Examples

AcrF8 (Pectobacterium phage ZF40)

Organism: BPZF4 (bacteriophage)
UniProt: H9C181
Status: COMPLETE

Key Findings:
- 92-amino acid protein that inhibits Type I-F CRISPR-Cas system
- Unique dual-targeting mechanism: binds BOTH Cas7f protein backbone AND crRNA scaffold
- Contacts nucleotides U[+21], U[+22], G[+23] of crRNA at <4Å distance
- Blocks R-loop formation and prevents target DNA recognition
- Cryo-EM structure at 3.42Å resolution (PMID:32170016)

Annotation Issues Identified:
- GO:0052170 (symbiont-mediated suppression of host innate immune response) - Too general
- Action: MODIFY → GO:0098672 (symbiont-mediated suppression of host CRISPR-cas system)
- Missing core function annotation for ribonucleoprotein complex binding

Proposed New GO Term:
- Name: CRISPR RNA binding anti-CRISPR activity
- Justification: Current terms don't distinguish between Acrs that only bind Cas proteins vs. those that directly contact crRNA. AcrF8 represents a unique class with dual protein-RNA binding.

Genes for Review

Priority 1: Structurally Characterized Acrs

Species	Gene	CRISPR Type	Status
BPZF4	AcrF8	Type I-F	COMPLETE

Priority 2: Other Acr Families

(To be identified - AcrIF, AcrIE, AcrIIA families)

Priority 3: Aca Regulators

(Anti-CRISPR associated proteins that regulate Acr expression)

Key Mechanisms to Annotate

1. Surveillance complex binding - GO:0043021 ribonucleoprotein complex binding
2. CRISPR-Cas suppression - GO:0098672 symbiont-mediated suppression of host CRISPR-cas system
3. DNA mimic function - Where Acrs structurally mimic DNA
4. crRNA binding - Direct RNA contacts (needs new term?)

Key References

• Bondy-Denomy J et al. (2013) Nature - Anti-CRISPR discovery
• Pawluk A et al. (2016) Cell - AcrIF mechanisms
• Wang J et al. (2020) Nat Commun - AcrF8/F9/F6 cryo-EM structures (PMID:32170016)
• Stanley SY et al. (2019) Cell - Aca repressor mechanisms (PMID:31474367)

Related Projects

• Phage-host interactions
• Bacterial immune systems

Slides

• Slides (Marp source: ANTI_CRISPR-slides.md) — AI generated

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STATUS

Completed Reviews

• [x] BPZF4/AcrF8 - Type I-F inhibitor with dual protein-RNA binding

Pending

• [ ] Identify additional Acr proteins in UniProt/QuickGO
• [ ] Review Aca regulator annotations
• [ ] Propose new GO terms for crRNA-binding mechanisms

Last updated: 2026-01-22

NOTES

2026-01-22

Project Creation

Created project to track anti-CRISPR protein annotations.

AcrF8 Highlights:
- Unique dual-targeting: binds both Cas7f protein AND crRNA directly
- Cryo-EM structure shows <4Å contacts with crRNA nucleotides
- Expression regulated by Aca2 repressor in negative feedback loop
- Proposed new GO term for crRNA-binding anti-CRISPR mechanism

The AcrF8 example demonstrates:
1. Need for more specific GO terms for CRISPR-Cas inhibition mechanisms
2. Value of structural biology in informing function annotations
3. Importance of distinguishing protein-only vs. protein+RNA binding mechanisms