Characterization of a stalled complex on the β-barrel assembly machine

Area of Science:

• Biochemistry
• Molecular Biology
• Membrane Protein Assembly

Background:

• Beta-barrel proteins are essential for cellular function and are assembled into membranes by a conserved machine.
• Understanding this assembly process is crucial, but lacks characterization of stable, partially folded intermediates.

Purpose of the Study:

• To investigate the folding mechanism of the Escherichia coli beta-barrel protein LptD, in complex with its lipoprotein LptE.
• To characterize stable, partially folded intermediates in the beta-barrel assembly pathway.

Main Methods:

• Slowed the folding of LptD with LptE to capture transient states.
• Identified and characterized a late-stage folding intermediate involving LptD, LptE, BamA, and BamD.

Main Results:

• A late-stage intermediate was identified where LptD is folded around LptE.
• This intermediate shows interaction with both BamA and BamD, key components of the beta-barrel assembly machine.
• A model is proposed where BamA and BamD cooperate to catalyze folding, with barrel closure and release as the final step.

Conclusions:

• The beta-barrel assembly machine (Bam) complex, comprising BamA and BamD, likely acts in concert to mediate beta-barrel protein folding.
• The assembly process, involving soluble proteins BamD and LptE, is proposed to initiate at the periplasmic membrane interface.