Expression and Purification of the Cas10-Csm Complex from Staphylococci.

CRISPR-Cas (Clustered regularly interspaced short palindromic repeats-CRISPR-associated proteins) is a class of prokaryotic immune systems that degrade foreign nucleic acids in a sequence-specific manner. These systems rely upon ribonucleoprotein complexes composed of Cas nucleases and small CRISPR RNAs (crRNAs). and are bacterial residents on human skin that are also leading causes of antibiotic resistant infections (Lowy, 1998; National Nosocomial Infections Surveillance, 2004; Otto, 2009). Many staphylococci possess Type III-A CRISPR-Cas systems (Marraffini and Sontheimer, 2008; Cao ., 2016), which have been shown to prevent plasmid transfer and protect against viral predators (Goldberg ., 2014; Hatoum-Aslan ., 2014; Samai ., 2015) in these organisms. Thus, gaining a mechanistic understanding of these systems in the native staphylococcal background can lead to important insights into the factors that impact the evolution and survival of these pathogens. Type III-A CRISPR-Cas systems encode a five-subunit effector complex called Cas10-Csm (Hatoum-Aslan ., 2013). Here, we describe a protocol for the expression and purification of Cas10-Csm from its native background or a heterologous background. The method consists of a two-step purification protocol involving Ni-affinity chromatography and a DNA affinity biotin pull-down, which together yield a pure preparation of the Cas10-Csm complex. This approach has been used previously to analyze the effects of mutations on Cas10-Csm complex integrity (Hatoum-Aslan ., 2014), crRNA formation (Hatoum-Aslan ., 2013), and to detect binding partners that directly interact with the core Cas10-Csm complex (Walker ., 2016). Importantly, this approach can be easily adapted for use in other species to probe and understand their native Type III-A CRISPR-Cas systems.