CRISPR RNA and anti-CRISPR protein binding to the Xanthomonas albilineans Csy1-Csy2 heterodimer in the type I-F CRISPR-Cas system

The crystal structure of the AcrF2 protein was solved to a resolution of 1.34 Å.

• The Csy1 and Csy2 proteins from Xanthomonas albilineans form a stable heterodimeric complex that specifically binds an 8-nucleotide 5'-handle of the crRNA.
• This heterodimer shows reduced affinity for a longer 28-nucleotide CRISPR repeat RNA containing the same 5'-handle sequence.
• AcrF2, an anti-CRISPR protein from a phage, tightly binds to the Csy1-Csy2 heterodimer, indicating it recognizes features common to these proteins.
• Neither Csy1 nor Csy2 alone can form a stable complex with AcrF2 and the 5'-handle RNA, highlighting the necessity of heterodimerization for binding.
• The findings contribute to understanding the sequence of events in forming the crRNA-guided surveillance complex and suggest broad specificity of Acr proteins against type I-F CRISPR-Cas systems.

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