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TamL is a Key Player of the Outer Membrane Homeostasis in Bacteroidota

Fabio Giovannercole1, Tom De Smet1, Miguel Ángel Vences-Guzmán2

  • 1Research Unit in Biology of Microorganisms (URBM), Namur Research Institute for life Sciences (Narilis), University of Namur, Namur, Belgium.

Journal of Molecular Biology
|March 5, 2025
PubMed

Insights

The Translocation and Assembly Module (TAM) complex is essential in Bacteroidota bacteria, with TamL and TamB crucial for maintaining outer membrane integrity. This study reveals TAM

Area of Science:

  • Microbiology
  • Bacterial Cell Biology
  • Protein Transport

Background:

  • The Translocation and Assembly Module (TAM) complex, comprising TamA and TamB, is vital for protein and lipid transport across bacterial membranes in Proteobacteria.
  • In Bacteroidota, TamA is replaced by TamL, a lipoprotein with a distinct structure, suggesting potentially different functions.
  • The role of TAM complex subunits in bacterial phyla beyond Proteobacteria remains largely uncharacterized.

Purpose of the Study:

  • To investigate the function of the TamL protein in the environmental Bacteroidota species, Flavobacterium johnsoniae.
  • To determine the essentiality and specific roles of TamL and TamB in F. johnsoniae.
  • To explore the conservation and potential variations of TAM components within the Bacteroidota phylum.

Main Methods:

  • Genetic analyses, including gene essentiality studies for TamL and TamB in F. johnsoniae.
  • Phenotypic characterization of TamL-depleted cells, assessing cell viability, morphology, and susceptibility to membrane-disrupting agents.
  • Proteomic and lipidomic analyses to evaluate changes in outer membrane protein and lipid composition.
  • Biochemical assays, such as pull-down experiments, to confirm interactions between TamL and TamB.

Main Results:

  • TamL and TamB were found to be essential for F. johnsoniae survival, unlike some homologs in Proteobacteria.
  • Depletion of TamL severely impaired outer membrane integrity, leading to reduced viability, altered cell shape, and increased sensitivity to detergents.
  • Elevated levels of outer membrane lipoproteins were observed upon TamL depletion, but overall β-barrel protein levels and lipid composition remained largely unchanged.
  • TamL was shown to co-purify with TamB, indicating a functional interaction within the F. johnsoniae TAM complex.

Conclusions:

  • The TAM complex, involving TamL and TamB, plays an essential role in maintaining outer membrane integrity in the Bacteroidota phylum.
  • TamL's function in F. johnsoniae differs from TamA in Proteobacteria, highlighting bacterial phylum-specific adaptations of the TAM system.
  • This study provides the first functional characterization of a TAM complex subunit outside of Proteobacteria, expanding our understanding of bacterial envelope biogenesis.
Keywords:
Bacteroidotabacterial membranesenvelope homeostasisouter membrane vesiclestranslocation and assembly module (TAM)

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