Crystal structure of Cmr5 from Pyrococcus furiosus and its functional implications

The bacterial acquired immune system consists of clustered regularly interspaced short palindromic repeats (CRISPRs) and CRIPSR-associated (Cas) genes, which include Cas-module repeat-associated mysterious proteins (Cmr). The six Cmr proteins of Pyrococcus furiosus (pfCmr1-pfCmr6) form a Cmr effector complex that functions against exogenous nucleic acid. Among the Cmr proteins, the role of pfCmr5 and its involvement in the complex's cleavage activity have been obscure. The elucidated pfCmr5 structure has two inserted α-helices compared with the other trimeric Cmr5 structure. However, pfCmr5 exists as a monomeric protein both in the crystalline state and in solution. In vitro assays indicate that pfCmr5 interacts with pfCmr4. These structural and biophysical data might help in understanding the complicated and ill-characterized Cmr effector complex. Structured summary of protein interactions: pfCmr4 and pfCmr5 bind by molecular sieving (View interaction) pfCmr4 and pfCmr4 bind by molecular sieving (View interaction) pfCmr5 and pfCmr4 bind by ion exchange chromatography (View interaction).