Structural asymmetry governs the assembly and GTPase activity of McrBC restriction complexes

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• McrBC complexes are essential for bacterial immunity, degrading foreign DNA.
• The AAA+ protein McrB acts as a motor, translocating along DNA and hydrolyzing GTP.
• McrC, a nuclease partner, stimulates McrB's GTPase activity.

Purpose of the Study:

• To elucidate the structural basis of McrBC complex function using cryo-electron microscopy (cryo-EM).
• To understand how asymmetry in McrB hexamers influences McrC binding and catalytic activity.
• To investigate conserved mechanisms in AAA+ motor proteins.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to determine the structures of Thermococcus gammatolerans McrB and McrBC, and E. coli McrBC.
• Structural analysis focused on the asymmetric arrangement of McrB subunits and interactions with McrC.

Main Results:

• McrB hexamers exhibit intrinsic asymmetry, crucial for McrC interaction at a single active site.
• An arginine/lysine-mediated hydrogen-bonding network facilitates optimal GTP hydrolysis by repositioning a key asparagine residue.
• Despite variations in DNA-binding domains, McrBC complexes utilize a conserved GTP-recognition mechanism.
• Complex asymmetry drives directional GTP hydrolysis through distinct inter-subunit interactions.

Conclusions:

• The study reveals how structural asymmetry in McrB motors dictates functional interactions with McrC for efficient DNA degradation.
• Insights into conserved AAA+ motor protein mechanisms and bacterial defense strategies are provided.
• The findings offer a molecular understanding of coordinated GTP hydrolysis in McrBC complexes.