Structure and Interaction of Type II Cas1 from Sterptococcus Pyogenes

Abstract

Clustered regularly interspaced short palindromic repeats (CRISPRs) and CRISPR-associated (Cas) proteins form a microbial immune system against invading foreign nucleic acids. Cas1 is one of the two universal Cas proteins present in nearly all types of CRISPR-Cas systems, and its role is implicated in new spacer acquisition during CRISPR-mediated immunity. Here we report the crystal structure of Streptococcus pyogenes Cas1 (SpCas1) in a type II CRISPR-Cas system and study its interaction with other S. pyogenes Cas proteins. Despite conserved domain folds, the SpCas1 structure exhibits a unique conformational state distinct from type I Cas1 structures, resulting in a more extensive dimerization interface, a more globular overall structure, and a disruption of a potential metal-binding site for catalysis. By using analytical size-exclusion chromatography and isothermal titration calorimetry, we demonstrate that SpCas1 directly interacts with SpCsn2 to form a complex. Taken together, our results provide structural information for a type II Cas1 protein, a previously uncharacterized Cas1 homologue, and lay a foundation for studying multiprotein Cas complexes functioning in the adaptive immunity of type II CRISPR-Cas systems.