Bacterial Flagella and Their Function

Overview

• Bacterial flagella are motorized structures that bacteria use to propel themselves.
• Rotobacter: Single flagellum.
• E. coli: Multiple flagella that rotate in bundles.
• Flagellum characteristics:
  • Helical filament: 20nm wide, up to 15 microns long.
  • Spin rate: ~100 times per second.

Structure of the Flagellum

• Three main parts:
  1. Filament: The long, helical structure visible outside the cell.
  2. Hook: Flexible connector between the filament and the basal body.
  3. Basal body: Anchors the flagellum to the cell wall; comprises a rod and rings:
     • Inner membrane rings
     • Peptidoglycan layer rings
     • Outer membrane rings (L and P rings in gram-negative bacteria)

Flagellar Motor

• Composed of the stator and rotor:
  • Stator: Stationary; attached to the peptidoglycan layer.
  • Rotor: Rotational component.
• Power source: Proton gradient across the cell membrane.
  • High proton concentration outside; low inside.
  • Proton flow: Through proteins MOTA and MOTB in the stator.

Mechanism of Action

• MOTB contains aspartic acid crucial for proton conductance.
• Proton movement: Binding to aspartic acid causes conformational changes in MOTA.
  • Results in two consecutive power strokes driving the rotor.
  • Protons are released into the cytoplasm.

Assembly of Flagella

• Begins with structures in the inner membrane:
  • MS ring: Formed by 26 subunits of Flea F protein.
  • Flea G, M, and N: Assemble under the MS ring, constitute the rotor.
• Extracellular assembly:
  • Flagellar proteins exported via a specific export pathway.
  • Mote A and Mote B: Form the stator, anchoring in the peptidoglycan layer.
• Rod assembly: Built upward through a hollow cylinder, guided by cap proteins.
• Outer membrane penetration:
  • L and P rings: Serve as bearings for the rod.
• Hook and Filament assembly:
  • Hook cap guides hook protein assembly.
  • Junction proteins connect hook and filament.
  • Filament proteins travel through a hollow channel to the distal end, building helically.
• Filament structure: 20,000 to 30,000 subunits.