Bacterial Transport Mechanisms

• Transport mechanisms


  Mechanisms cells use to accumulate solutes (most of which are vital nutrients) to levels higher than those in their habitats
• Transport mechanisms


  • Continuous nutrient uptake and waste export are needed to fuel metabolism and support growth
  • Polar and charged molecules cannot easily diffuse through the membrane
  • Require energy in some form
  • Membrane-spanning proteins show significant similarities on amino acid sequence
  • Require a conformational change in the membrane protein following the binding of its solute

  
• Membrane transporters


  Consist of 12 alpha helices that weave back and forth through the membrane to form a channel

  
• Transport systems

  • Saturation effect
  • High specificity
  • Biosynthesis is typically highly regulated
• Uniport


  Transport of molecules unidirectionally (either in or out) across the membrane
• Symport


  Cotransport - transport of molecule along with another substance (typically a proton)
• Antiport


  Transport of one molecule into the cell while simultaneously transporting a 2nd molecule out of the cell
• Simple transport


  Driven by proton motive force, consists only of a membrane-spanning transport protein

  
• Simple transport

  • Lac permease of E. coli
• Lac permease


  Transports lactose along with H+ (symporter), permits energy-driven accumulation of lactose in the cytoplasm against the concentration gradient

  
• Group translocation


  Involves a series of proteins in the transport event, the substance transported is chemically modified during its uptake across the membrane (driven by phosphoenolpyruvate)

  
• Example of Group translocation

  • Phosphotransferase system
• Phosphotransferase system


  Consists of family of proteins necessary in transport any given sugar, prior to sugar transport the proteins are alternately phosphorylated and dephosphorylated in a cascading manner until the actual transporter phosphorylates the sugar during the transport event
• ABC transporters


  For the uptake of organic compounds, inorganic nutrients, and trace materials, three components: substrate binding protein, membrane-integrated transporter, and ATP-hydrolyzing protein

  
• ABC transporters


  • Periplasmic binding proteins have high substrate affinity even at extremely low concentrations, once the substrate is bound the binding protein interacts with its respective membrane transporter to transport the substrate into the cell driven by ATP hydrolysis
• General secretory pathway (GSP) = Sec system


  Classified according to how folding of peptides are prevented before excretion, SRP and SecB pathway
• Signal sequence/peptide


  Unique N-terminal sequence of proteins transported through GSP, has three regions: basic region at N-terminal end, central hydrophobic region, and recognition site for peptidase

  
• SecB Pathway


  Molecular chaperone: SecB, Protein-conducting channel (translocon): SecY complex

  
• SRP Pathway


  1. N-terminal hydrophobic signal peptide is bound by the SRP at the initial stage of translation, FtsY: membrane-bound receptor, SecA: ATPase
• Twin-arginine translocation (TAT) pathway


  Embeds cytoplasmic membrane proteins containing cofactors, translocates largely folded structures, may also translocate other proteins that might fold too rapidly for Sec system to handle
• TAT signal sequence


  Longer and less hydrophobic than the Sec signal sequence, twin-arginine motif, ATP is not required but ∆pH is consumed to provide energy
• ATP-binding cassette (ABC) pathway


  Facilitates the transport of various nutrients including sugars, amino acids, and ions, LPS, capsular polysaccharides, extrusion of noxious substances, targeting of membrane and surface structures
• Chaperone/usher pathway


  Exports cell surface proteins associated with virulence, periplasmic chaperone (PapD), usher targets the PapD-peptide complex to OM, energy is not needed, pili formation
• Type I pathway: ABC pathway


  Exports extracellular proteins such as proteases, lipases, and toxins, GSP independent, membrane fusion protein (MFP), outer membrane protein (OMP), hydrolysis of ATP
• Type II pathway


  Wide variety of proteins, e.g. pullulanase by Klebsiella oxytoca, secreton - 12 proteins
• Type III pathway


  Injection of anti-host factors directly into the host cytoplasm, GSP independent, 20 secretion components, needle (tube-like structure), ATP hydrolysis
• Type IV pathway


  Injection of anti-host factors directly into the host cytoplasm, Legionella pneumophila, Helicobacter pylori, similar mechanism with type III but not homologous
• Type V pathway


  Proteases, toxins, adhesins, invasins, three domains: N-terminal signal sequence, internal passenger (functional), C-terminal beta-domain
• Sec System


  General Secretory Pathway (GSP)
• Basic region at N-terminal end


  positively charged at physiological pH values, attaches to the negatively charged membrane phospholipid
• Central Hydrophobic Region 


  inserts itself into the cytoplasmic membrane; facilitates the transport of the main peptide
• Recognition Site for Peptidase


  cleaved by signal peptidase at the membrane during or after transport
• Compare and contrast Type III and Type IV pathway?

  Both injects anti-host factors but Type IV pathway is not homologous