Comparative structural analysis of TonB-dependent outer membrane transporters: implications for the transport cycle

Area of Science:

• Structural biology
• Microbial physiology
• Biochemistry

Background:

• TonB-dependent outer membrane transporters (TBDTs) facilitate the translocation of organometallic substrates across the Gram-negative bacterial outer membrane.
• X-ray crystallography has revealed the structures of four distinct TBDTs, consistently showing a 22-stranded beta-barrel surrounding a hatch domain.

Purpose of the Study:

• To analyze the conserved structural motifs and interfaces within TBDTs.
• To compare TBDT interface properties with other protein-protein interactions.
• To infer the functional implications of these interfaces for the transport mechanism.

Main Methods:

• Structure-based sequence alignment of four TBDT structures.
• Analysis of interfacial properties between the barrel and hatch domains.
• Comparison of TBDT interfaces to known protein-protein interfaces.

Main Results:

• Highly conserved motifs were identified in both the hatch and barrel domains of TBDTs, consistently interacting.
• TBDT interfaces are extensively hydrated, with water molecules participating in hydrogen bonding and bridging.
• The properties of these interfaces resemble those of obligate transient protein complexes, indicating suitability for conformational changes.

Conclusions:

• TBDTs possess interfaces that readily accommodate significant conformational changes and movement of the hatch domain during transport.
• These structural dynamics likely play a crucial role in the active transport cycle.
• The energy-coupling protein TonB may exert modest forces to induce these necessary conformational changes.