Structural and functional study of Streptococcus pyogenes Csn2, CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats)-associated protein

Abstract

The CRISPR-Cas (clustered regularly interspaced short palindromic repeats-CRISPR-associated proteins) system functions as an adaptive immune system that protects prokaryotic cells from invading genomic nucleic acids, such as bacteriophages and conjugative plasmids. Csn2 is a Nmeni subtype-specific Cas protein, recently classified as type Ⅱ–A, required for new spacer acquisition in the adaptation process of this immune system. In this study, the crystal structure of Streptococcus pyogenes Csn2 revealed a ring-like quaternary structure including calcium binding sites. Also, the Csn2 has a non-specific double-stranded (ds-) DNA-binding activity. In summary, the results support the suggestions of Csn2 functioning at the CRISPR adaptation stage.