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Mechanisms of solute transport through outer membrane porins: burning down the house
1Department of Chemistry and Biochemistry, University of Oklahoma, 620 Parrington Oval, Norman, Oklahoma 73019, USA.
Abstract:
Porins mediate the uptake of nutrients across the outer membrane of Gram-negative bacteria. For general porins like OmpF, electrophysicoloigcal experiments now establish that the charged residues within their channels primarily modulate pore selectivity, rather than voltage-gated switching between open and closed states. Recent studies on the maltoporin, LamB, solidify the importance of its 'greasy slide' aromatic residues during sugar transport, and suggest the involvement of L9, in the exterior vestibule, as the initial maltodextrin binding site. The application of biophysical methodologies to the TonB-dependent porin, FepA, ostensibly reveal the opening and closing of its channel during ligand uptake, a phenomenon that was predicted but not previously demonstrated.
Insights
Charged residues in bacterial porins OmpF modulate selectivity, not gating. Maltoporin LamB uses aromatic residues and L9 for sugar transport, while TonB-dependent FepA shows channel gating during nutrient uptake.
Area of Science:
- Microbiology
- Structural Biology
- Biophysics
Background:
- Porins are essential outer membrane proteins in Gram-negative bacteria, facilitating nutrient transport.
- Understanding porin function is crucial for developing novel antibacterial strategies.
- Previous models proposed voltage-gated mechanisms for some porins, but experimental evidence was limited.
Purpose of the Study:
- To elucidate the precise mechanisms governing nutrient transport through bacterial porins.
- To differentiate between charge-based selectivity and voltage-gating in general porins.
- To investigate the roles of specific residues and structural features in porin function.
Main Methods:
- Electrophysiological experiments were employed to study OmpF porin.
- Biophysical methodologies were applied to investigate the maltoporin LamB and the TonB-dependent porin FepA.
- Structural analysis focused on aromatic residues and specific binding sites.
Main Results:
- Electrophysiology confirmed that charged residues in OmpF channels primarily determine selectivity, not voltage-gating.
- Studies on LamB highlighted the significance of its 'greasy slide' aromatic residues for maltodextrin transport.
- L9 was identified as a potential initial binding site for maltodextrin in LamB.
- Biophysical data for FepA provided the first experimental demonstration of channel opening and closing during ligand uptake.
Conclusions:
- Bacterial porin function is diverse, involving distinct mechanisms for different porin types.
- Charge-based selectivity is a key feature of general porins like OmpF.
- Specific structural elements, such as aromatic residues and vestibule sites, are critical for substrate recognition and transport.
- TonB-dependent porins exhibit dynamic gating mechanisms essential for nutrient acquisition.