Bacterial Physiology (Micr430)

1. Bacterial Physiology (Micr430) Lecture 11 Protein Transport (Text Chapter: 17)

2. Protein Transport: 3 modes • Translocation – transport of proteins into or through a membrane • Export – the protein is translocated into the periplasm of a Gram negative bacterium • Secretion – a protein is transported to the extracellular medium, into another cell, or to the bacterial cell surface.

3. The Sec System • Proteins destined for cell membrane, periplasm or outer membrane use Sec system. • Sec System has four components: • A leader peptide; • A chaperone protein; • A membrane-bound complex of three proteins (SecYEG) • A cytoplasmic ATPase (SecA)

4. Leader Peptides of Exported Proteins Fig. 17.1

5. A model Fig. 17.2

6. Translocation of membrane-bound proteins • For some proteins destined to be residing in cell membrane, the transport is also Sec dependent and occurs in similar fashion. • Internal hydrophobic regions of the protein stop “total” translocation and anchor the protein into the membrane.

7. A model Fig. 17.3

8. Extracellular Protein Secretion • A variety of different proteins are secreted out of the cell: • Hydrolytic enzymes: proteases, nucleases • Nonhydrolytic enzymes; such as cholera toxin, diphtheria toxin. • Structural proteins such as pilin and flagellar proteins • Virulence proteins secreted directly into host cells

9. Extracellular Protein Secretion • There are six major secretion systems known for Gram negative bacteria; we will concentrate on Type I, Type II and Type III. • Type II system is Sec dependent • Types I and III are Sec independent

10. An Overview Fig. 17.4

11. The Type I Pathway • Examples of proteins secreted using this pathway: hemolysin and proteases • Protein characteristics: • No leader sequences but carboxyl terminus of the proteins is recognized by the secretion apparatus • Do not require sec genes • Proteins are directly secreted outside the cell without ever entering the periplasm

12. The Type I Pathway • Three proteins form the secretion apparatus in this pathway: • An ABC transporter (also known as ABC exporters) (example, HlyB) has ATPase activity to generate energy for secretion • The membrane fusion protein (MFP) (example, HlyD) is anchored in the inner membrane and has a periplasmic domain that is believed to connect to the OMP. • The outer membrane protein (OMP) (example, TolC)

13. The Type III Pathway • Type III (three) secretion systems (TTSS) are also Sec-independent. • These systems form an apparatus used for the injection of virulence proteins into eukaryotic host cells in some Gram negative bacteria such as Yersinia spp., Salmonella spp., Pseudomonas spp.

14. A Model for Type III pathway Fig. 17.5

15. TTSS and Flagellin Secretion • Type III appears to have evolved from flagellin secretion system based on homology of many proteins of these two systems.

16. Fig. 17.6

17. The Type II Pathway • Type II pathway appears to be the major route by which proteins are secreted by Gram negative bacteria. • This pathway secrete proteins in two stages: • Stage 1, proteins are translocated into periplasm; Sec pathway is used in this stage • Stage 2, proteins from periplasm are secreted across the outer membrane to the external environment

18. A Model for Type II pathway Fig. 17.7