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Structural insights into type-I and type-II Lamassu antiphage systems.

Ming Li1, Xiaolong Zhao1, Xingyu Zhao2

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Bacteria use Lamassu systems, featuring SMC proteins, to fight phages. This study reveals how Lamassu complexes inactivate and activate the LmuA nuclease to defend against viral DNA.

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Area of Science:

  • Microbiology
  • Structural Biology
  • Immunology

Background:

  • Bacteria possess diverse defense mechanisms against bacteriophage infections.
  • The Lamassu system is a prokaryotic immune system utilizing SMC superfamily proteins (LmuB) and effectors (LmuA).
  • The precise mechanism of Lamassu-mediated antiphage defense remains largely uncharacterized.

Purpose of the Study:

  • To elucidate the structural basis and mechanism of the Lamassu antiphage defense system.
  • To investigate the role of SMC proteins in bacterial immunity.
  • To understand how the LmuA effector is regulated within the Lamassu complex.

Main Methods:

  • Cryo-electron microscopy (cryo-EM) to determine the structures of Type-I and Type-II Lamassu complexes.
  • Biochemical analyses to assess protein function and interactions.
  • Structural and biochemical data integration to reveal functional mechanisms.

Main Results:

  • Determined cryo-EM structures of Lamassu complexes from *Bacillus cellulasensis* and *Vibrio cholerae* in apo and dsDNA-bound states.
  • Revealed an unexpected stoichiometry and topological architecture distinct from canonical SMC complexes.
  • Demonstrated that the LmuA nuclease is sequestered in an inactive monomeric form and becomes active upon sensing foreign DNA ends, forming a tetramer.

Conclusions:

  • Elucidated the mechanism of Lamassu-mediated antiphage defense, involving sensing viral DNA and activating nuclease effectors.
  • Highlighted the crucial role of SMC proteins in bacterial adaptive immunity.
  • Provided insights into the regulation and activation of the LmuA effector during phage infection.