Abstract
CRISPR (cluster of regularly interspaced palindromic repeats) is a prokaryotic adaptive defence system, providing immunity against mobile genetic elements such as viruses. Genomically encoded crRNA (CRISPR RNA) is used by Cas (CRISPR-associated) proteins to target and subsequently degrade nucleic acids of invading entities in a sequence-dependent manner. The process is known as ‘interference’. In the present review we cover recent progress on the structural biology of the CRISPR/Cas system, focusing on the Cas proteins and complexes that catalyse crRNA biogenesis and interference. Structural studies have helped in the elucidation of key mechanisms, including the recognition and cleavage of crRNA by the Cas6 and Cas5 proteins, where remarkable diversity at the level of both substrate recognition and catalysis has become apparent. The RNA-binding RAMP (repeat-associated mysterious protein) domain is present in the Cas5, Cas6, Cas7 and Cmr3 protein families and RAMP-like domains are found in Cas2 and Cas10. Structural analysis has also revealed an evolutionary link between the small subunits of the type I and type III-B interference complexes. Future studies of the interference complexes and their constituent components will transform our understanding of the system.
Highlights
cluster of regularly interspaced palindromic repeats (CRISPR) are a prokaryotic defence mechanism against viral infection and horizontal gene transfer
The structural biology of the CRISPR system provides a wealth of information on the evolution and mechanisms of the proteins involved
The repeat-associated mysterious protein (RAMP) domains, present in the Cas2, Cas5, Cas6, Cas7, Cas10 and Cmr3 families, are the leitmotif of the system, providing RNA-binding and -cleavage functionalities that are central to the process
Summary
CRISPRs (cluster of regularly interspaced palindromic repeats) are a prokaryotic defence mechanism against viral infection and horizontal gene transfer. These crRNAs are subsequently bound by complexes of Cas proteins and used to target homologous foreign dsDNA (doublestranded DNA) or ssRNA (single-stranded RNA) for nucleolytic degradation during CRISPR interference (Figure 1) [8,11].
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