Recent advances in noncanonical inhibition mechanisms of anti-CRISPR proteins

The CRISPR-Cas system constitutes an adaptive immune mechanism in prokaryotes that defends against mobile genetic elements. Within the perpetual co-evolutionary arms race between bacteria and their viral predators, bacteriophages encode anti-CRISPR (Acr) proteins that use sophisticated molecular strategies to sabotage CRISPR-Cas function. While canonical Acr proteins rely on steric blockade of Cas effectors, recent discoveries reveal unprecedented noncanonical mechanisms spanning CRISPR immunity stages. This review synthesizes recent mechanistic advances in this field since 2023, highlighting the expansion of noncanonical inhibition mechanisms beyond type I to include types II, V, and VI, as well as novel Acr interventions targeting multiple functional stages, such as spacer acquisition, translation-coupled inhibition, complex assembly/disassembly, and R-loop DNA binding. Structural insights demonstrate how Acr proteins achieve substoichiometric inhibition via conformational hijacking, catalytic repurposing, and molecular mimicry. Forged by the intense selective pressure of the phage-host conflict, these molecular innovations represent both remarkable evolutionary adaptations and versatile precision tools. They enable spatiotemporal control of CRISPR technologies, from engineered off-switches to diagnostic reset mechanisms, while posing critical challenges for therapeutic safety and microbiome management.