Its own architect: Flipping cardiolipin synthase.

Area of Science:

• Membrane biophysics
• Molecular biology
• Cellular biochemistry

Background:

• Biological membranes exhibit lipid asymmetry, crucial for cellular functions.
• The mechanism maintaining asymmetry in Escherichia coli's inner membrane (IM) remains elusive.
• Current models often assume active lipid transport mechanisms.

Purpose of the Study:

• To elucidate the molecular mechanism responsible for maintaining lipid asymmetry in the E. coli IM.
• To investigate the role of cardiolipin (CL) and cardiolipin synthase (ClsA) in membrane lipid distribution.
• To challenge the dogma of active lipid maintenance by proposing a self-organization model.

Main Methods:

• Engineering a conditionally lethal phosphatidylethanolamine (PE)-deficient mutant of E. coli.
• Utilizing osmotic down-shock to induce topological changes in ClsA.
• Observing the localization and activity of ClsA under varying PE concentrations.

Main Results:

• Cardiolipin (CL) presence in the periplasmic leaflet of the IM is essential for cell viability in PE-deficient mutants.
• CL synthase (ClsA) undergoes a topological inversion, relocating its catalytic domain to the periplasmic side upon PE depletion.
• This enzyme reorientation ensures the supply of nonbilayer-prone CL to the periplasmic leaflet, maintaining membrane integrity.
• Osmotic down-shock induces ClsA topological inversion in wild-type cells, indicating biological relevance.

Conclusions:

• A flippase-less mechanism involving enzyme self-organization maintains membrane lipid asymmetry.
• The topological plasticity of lipid-synthesizing enzymes like ClsA is key to lipid distribution.
• This study reveals a novel mechanism for generating and maintaining membrane lipid asymmetry in living cells.