Solution structure and elevator mechanism of the membrane electron transporter CcdA

Area of Science:

• Biochemistry
• Structural Biology
• Microbiology

Background:

• Membrane oxidoreductase CcdA is crucial for transferring reducing equivalents from the bacterial cytoplasm to the envelope.
• The electron transport mechanism of CcdA across the membrane, involving a single pair of cysteines, remains largely uncharacterized.

Purpose of the Study:

• To determine the structure of Thermus thermophilus CcdA (TtCcdA) in an oxidized, outward-facing state.
• To elucidate the transport mechanism of CcdA through structural and computational modeling.

Main Methods:

• Nuclear Magnetic Resonance (NMR) spectroscopy was used to determine the structure of TtCcdA.
• Computational modeling was employed to predict and validate an inward-facing state.
• Experimental validation confirmed the modeled inward-facing conformation.

Main Results:

• The NMR structure revealed TtCcdA comprises two inverted 3-transmembrane helix repeats (2x3-TM).
• Both outward-facing (oxidized) and inward-facing (modeled and validated) states were characterized.
• An elevator-type movement mechanism was proposed for shuttling reactive cysteines across the membrane.

Conclusions:

• The determined structure and proposed mechanism provide a framework for understanding CcdA function.
• CcdA's structural insights are relevant to other LysE superfamily transporters.
• Comparisons with other transporters illuminate general principles of membrane transporter architecture and mechanism.