Are there different CRISPR-associated system (Cas) proteins?

In bacterial and archaeal genomes, at least 45 distinct protein families have been identified to be associated with CRISPR loci.3 These CRISPR-associated system (Cas) proteins are used to classify CRISPR/Cas systems as Type I, II, or III. Type I systems are marked by the presence of cas3 (a single-stranded DNA nuclease and ATP-dependent helicase4), Type II by cas9 (a dual RNA-guided DNA endonuclease5), and Type III by cas10 (function unclear2). All CRISPR/Cas systems contain cas1 (a sequence specificity-free metal-dependent DNAse) and cas2 (a metal-dependent endoribonuclease). Both of these enzymes are involved in spacer acquisition.1

1. K.S. Makarova, et al., “Unification of Cas protein families and a simple scenario for the origin and evolution of CRISPR-Cas systems,” Biol Direct 6:38, 2011.
2. D. Rath, et al., “The CRISPR-Cas immune system: biology, mechanisms and applications,” Biochimie 117:119-128, 2015.
3. D.H. Haft, et al., “A Guild of 45 CRISPR-Associated (Cas) Protein Families and Multiple CRISPR/Cas Subtypes Exist in Prokaryotic Genomes,” PLoS Comput Biol 1(6):e60, 2005.
4. T. Sinkunas, et al., “Cas3 is a single-stranded DNA nuclease and ATP-dependent helicase in the CRISPR/Cas immune system,” EMBO J 30(7):1335-1342, 2011.
5. M. Jinek, et al., “A programmable dual-RNA-guided DNA endonuclease in adaptive bacterial immunity,” Science 337(6096):816-821, 2012.