RESUMO
Mobile genetic elements in bacteria are neutralized by a system based on clustered regularly interspaced short palindromic repeats (CRISPRs) and CRISPR-associated (Cas) proteins. Type I CRISPR-Cas systems use a "Cascade" ribonucleoprotein complex to guide RNA specifically to complementary sequence in invader double-stranded DNA (dsDNA), a process called "interference." After target recognition by Cascade, formation of an R-loop triggers recruitment of a Cas3 nuclease-helicase, completing the interference process by destroying the invader dsDNA. To elucidate the molecular mechanism of CRISPR interference, we analyzed crystal structures of Cas3 from the bacterium Thermobaculum terrenum, with and without a bound ATP analog. The structures reveal a histidine-aspartate (HD)-type nuclease domain fused to superfamily-2 (SF2) helicase domains and a distinct C-terminal domain. Binding of ATP analog at the interface of the SF2 helicase RecA-like domains rearranges a motif V with implications for the enzyme mechanism. The HD-nucleolytic site contains two metal ions that are positioned at the end of a proposed nucleic acid-binding tunnel running through the SF2 helicase structure. This structural alignment suggests a mechanism for 3' to 5' nucleolytic processing of the displaced strand of invader DNA that is coordinated with ATP-dependent 3' to 5' translocation of Cas3 along DNA. In agreement with biochemical studies, the presented Cas3 structures reveal important mechanistic details on the neutralization of genetic invaders by type I CRISPR-Cas systems.
Assuntos
Bactérias/enzimologia , Proteínas de Bactérias/química , Proteínas Associadas a CRISPR/química , Sistemas CRISPR-Cas/fisiologia , DNA Helicases/química , DNA Bacteriano/metabolismo , Desoxirribonucleases/química , Sequências Repetitivas Dispersas , Trifosfato de Adenosina/metabolismo , Motivos de Aminoácidos , Sequência de Aminoácidos , Bactérias/genética , Bactérias/imunologia , Proteínas de Bactérias/genética , Proteínas de Bactérias/metabolismo , Proteínas Associadas a CRISPR/genética , Proteínas Associadas a CRISPR/metabolismo , Repetições Palindrômicas Curtas Agrupadas e Regularmente Espaçadas/fisiologia , Cristalografia por Raios X , DNA/metabolismo , DNA Helicases/genética , DNA Helicases/metabolismo , DNA de Cadeia Simples/metabolismo , Desoxirribonucleases/genética , Desoxirribonucleases/metabolismo , Interações Hospedeiro-Patógeno , Magnésio/metabolismo , Manganês/metabolismo , Modelos Moleculares , Dados de Sequência Molecular , Mutagênese Sítio-Dirigida , Ligação Proteica , Conformação Proteica , Estrutura Terciária de Proteína , RNA Bacteriano/metabolismo , Proteínas Recombinantes de Fusão/química , Proteínas Recombinantes de Fusão/genética , Proteínas Recombinantes de Fusão/metabolismo , Alinhamento de Sequência , Homologia de Sequência de AminoácidosRESUMO
OBJECTIVE: To investigate the role of nuclear factor-ΚB (NF-ΚB) in severe pneumonia and observe the effects of Xuebijing injection in its treatment. METHODS: Thirty hospitalized patients with severe pneumonia were divided into the routine therapy group (n=14) and Xuebijing therapy group (n=16) in whom with Xuebijing injection 100 ml was given once daily for 7 days besides routine therapies, according to the random numeral. The DNA binding activity of NF-ΚB in human monocytes was detected before and 3 days and 7 days after administration, the contents of tumor necrosis factor-α (TNF-α), procalcitonin (PCT) and C-reactive protein (CRP) were determined, and the changes in coagulatory and fibrinolytic parameters were assayed at the same time. Acute physiology and chronic health evaluationII (APACHEII) score was also recorded. Ten healthy volunteers served as the healthy control group. RESULTS: The DNA binding activities of NF-ΚB, the contents of TNF-α, PCT, CRP, fibrinogen (Fib), D-dimer in hospitalized subjects with severe pneumonia were higher before treatment than those in healthy control group, while the prothrombin time (PT), thrombin time (TT) were significantly lower (P<0.05 or P<0.01). Compared with the routine therapy group, the DNA binding activity of NF-ΚB (grey level) at the 7 days (66.60±36.23 vs. 79.90±39.11) was notably decreased in Xuebijing therapy group; the levels of TNF-α (ng/L, 25.81±11.67 vs. 33.78±13.36), PCT (µg/L, 1.91±1.09 vs. 2.96±1.80), CRP (mg/L, 20.01±7.21 vs. 26.59±10.66), Fib (g/L, 4.02±1.26 vs. 5.09±1.43), D-dimer (mg/L, 0.24±0.06 vs. 0.31±0.11) were significantly lower in Xuebijing therapy group, and APACHEII score (15.81±3.47 vs. 17.93±3.05) was obviously lowered (all P<0.05). There was statistical difference of the TT (s) between two groups at 3 days (15.68±1.89 vs. 14.65±1.33,P<0.05). There was a significant positive correlation between NF-ΚB DNA binding activity and the levels of TNF-α (r(1)=0.373, r(2)=0.362, r(3)=0.419), PCT (r (1)=0.800, r(2)=0.716, r(3)=0.920) or CRP (r(1)=0.368, r(2)=0.441, r(3)=0.366, all P<0.05) before and 3 days and 7 days after the treatment. CONCLUSION: NF-ΚB activation and coagulopathy were observed in patients with severe pneumonia, and NF-ΚB was involved in the process of inflammatory response. Inflammatory response was partly alleviated by Xuebijing injection. These effects of Xuebijing injection may be mediated by inhibition of the activation of NF-ΚB and its anticoagulation property.