Overview
This lecture covered specialized transport systems in bacteria, focusing on the TonB-dependent transport apparatus in Gram-negative bacteria, and introduced key steps in bacterial cell division and binary fission.
Specialized Transport in Bacteria
- TonB-dependent transport is unique to Gram-negative bacteria, moving specific substrates across both outer and inner membranes.
- ABC (ATP-Binding Cassette) family transporters use ATP hydrolysis to drive solute transport across membranes.
- ABC transporters consist of a membrane pore domain, an ATP hydrolysis domain, and a solute-binding protein.
- The solute-binding protein delivers the substrate to the permease, triggering ATP hydrolysis and substrate uptake.
- Secondary active transporters (symporters and antiporters) use ion gradients (proton, sodium, potassium) to move solutes.
- Symporters move solute and ion in the same direction; antiporters move them in opposite directions, based on environmental concentrations.
TonB-Dependent Transporters
- TonB, ExbB, and ExbD proteins form a conserved complex enabling outer membrane transport in Gram-negative bacteria.
- Initial substrate transport across the outer membrane is powered by the proton motive force via TonB-mediated conformational changes.
- After crossing the outer membrane, the substrate is picked up by a periplasmic solute-binding protein for ABC transporter-mediated inner membrane uptake.
- The TonB system is shared among various specific substrate transport systems by recognizing the TonB box sequence on substrate-specific receptors.
- Transport cycles involve TonB interacting with outer membrane receptors to open the channel, then being recaptured by ExbB/ExbD.
Group Translocation: PTS System
- Group translocation uses phosphoenolpyruvate (PEP) as a phosphate donor to modify sugars during import (e.g., glucose, mannitol).
- The PTS system consists of shared proteins (Enzyme I, HPr) and substrate-specific Enzymes II (IIA, IIB, IIC).
- Only one sugar is preferentially imported and phosphorylated at a time, based on environmental availability and enzyme affinity.
- Phosphate transfer proceeds from PEP → E1 → HPr → EIIA → EIIB → sugar.
Overview of Bacterial Transport Types
- Facilitated and simple diffusion follow concentration gradients, while active transport uses energy (ATP, ion gradients, or phosphate intermediates) to move solutes.
- All transport systems are selective for their substrates.
- Aquaporins facilitate water movement across membranes and can be regulated by the cell.
Bacterial Cell Division and Binary Fission
- Most bacteria divide by binary fission, producing two identical daughter cells.
- Cell division involves genome replication, separation, and coordinated cell wall and membrane synthesis.
- The FtsZ protein forms the constricting ring at midcell, essential for division.
- Systems like the Min system ensure FtsZ assembles only at the center.
- Plasmid and genomic partitioning systems ensure even genetic material distribution to daughter cells.
- Cell envelope synthesis (peptidoglycan, lipids) is tightly coordinated with division.
Key Terms & Definitions
- ABC transporter — Membrane protein complex that uses ATP hydrolysis to transport solutes.
- TonB-dependent transporter — Outer membrane system in Gram-negatives using TonB, ExbB, and ExbD to import substrates.
- Symporter — Transports solute and ion together in the same direction.
- Antiporter — Transports solute and ion in opposite directions.
- Proton motive force (PMF) — Energy from proton gradient across a membrane.
- PTS system — Phosphotransferase group translocation system using PEP-derived phosphate to import and modify sugars.
- FtsZ protein — Tubulin-like protein forming the division ring during bacterial cell division.
- Min system — Protein system preventing incorrect FtsZ ring placement.
- Aquaporin — Membrane channel protein facilitating water transport.
Action Items / Next Steps
- Complete the homework assignment on ELC by 11 p.m. tomorrow.
- Read the assigned paper before Wednesday’s class for discussion and homework questions.
- Review the seminar series PDF and attend a seminar for extra credit if interested (confirm by email if attending a non-microbiology seminar).