Transcript medmicro4-weapons delivery – G+

SBM 2044: Lecture 4
Weapons delivery & deployment
(Part II)
Secretion & targeting
of protein virulence factors
in Gram-positive bacteria
Sec-dependant General secretion pathway (GSP)
Gram-negative bacteria
Proteins reach periplasm, but
OM is additional barrier need other mechanisms to
get protein out thro’ OM.
(Types I - V secretion)
Gram-positive bacteria
Sufficient to get protein
out. In this case, other
mechanisms needed to
retain wall - associated
proteins
OM
IM
Type II
secretion
sec
sec
Signal-peptide
Targeting secreted proteins to Gram-positive cell walls
Four distinct mechanisms identified to date:
Rare:
•
Binding to wall teichoic acid
•
Binding to membrane anchored LTA
More widespread:
•
Lipoprotein ‘anchors’
• C-terminal wall-associating signals
1. Binding to cell-wall teichoic acid
Streptococcus pneumoniae and Streptococcus suis
Pneumococcal surface protein A (PspA)
Pneumococcal autolysin (LytA)
S. suis autolysin- [homologous to pneumococcal LytA]
C-terminal ends share homologous choline-binding
domains – enable binding to TA of these species
The structure of teichoic acid:
Polymer of either Glycerol phosphate or Ribitol phosphate, with
various substituents (R)
poly-ribitol phosphate
O
O
P
O
O
H
H
H
H
H
C
C
C
C
C
H
O
OH O
H
R
O
O
P
O
R’
H
O
C
H
n
In most species studied to date
R = D-alanine
R’ = N-acetylglucosamine
In S. pneumoniae and S. suis
R = phosphodiester linked choline - chemically more stable than
ester-linked D-Ala
2. Binding to membrane anchored LTA
Single example recognised only recently
- InlB of Listeria monocytogenes – has C-terminal
domain that ‘targets’ LTA – mechanism??
3. Lipoproteins
• attached at outer surface of cytoplasmic membrane by a
lipid anchor
Examples include penicillinase in S. aureus
• Similar mechanisms used in both Gram + & Gram .
Distinctive N-terminal signal peptides
recognized by
distinct Sec apparatus with specialized signal
peptidase (called signal peptidase II)
Lipoprotein signal peptides
NShort
hydrophobic
1-3 positively
sequence
charged a.a.
Signal peptidase II
cleavage site
-Leu-x-y- Cysx and y usually
small, uncharged
residues
A diglyceride is attached
to the N-terminal Cys of
the mature protein
Diglyceride
Contrast with ‘typical’ GSP secretion signal-peptide ( Lecture 3 )
4. ‘Sorting’ via C-terminal wall-associating signals
Vast majority of Gram + wall-associated proteins share
structurally similar C-terminal wall-associating signals
Hydrophobic /Charged ‘tail’
membrane ‘anchor’
-C
Pro-rich region
LPxTG
motif
15 - 20 hydrophobic
residues
5 - 10
mostly
charged
C-terminal wall-associating signals
Studies of S. aureus Protein A,
showed that membrane ‘anchor’
plays a transient role in a more
complex wall-associating pathway
Pro-rich
‘flexible’
wall-spanning
Hydrophobic
Charged ‘tail’
Membrane ‘anchor’
+
+
Care: do not be misled by some textbooks/reviews which say proteins
anchored in membrane.
C
N-terminal signal peptide
N
Wall-associating
signal
Signal
peptidase
wall-associated
‘Sortase’
Cleavage at
LPxTG
N
mRNA
G
Cross-linked
to cell-wall
Some, but not
C
Majority
Minority
necessarily all,
‘cleaved’
simply
covalently
‘anchored’? at LPxTG
linked to wall
(e.g. ActA in Listeria)
(e.g. InaA, Prot. A)
Retaining secreted proteins in Gram-positive cell walls
1. Binding to wall teichoic acid
Limited to a very few species (e.g. S. pneumoniae, S. suis)
2. Binding to membrane anchored LTA
Single example recognised only recently (InlB of Listeria
monocytogenes)
3. Lipoprotein ‘anchors’
A minority of wall-associated proteins in many species anchored
to outer surface of cell membrane via an N-terminal lipid anchor
4. C-terminal wall-associating signals
Vast majority of wall-associated proteins studied to date
share structurally similar C-terminal wall-associating signals
Retaining proteins at Gram-negative cell-surfaces
First step: Sec-dependent secretion to periplasm (GSP)
Then:
• Targeting of integral OM proteins - OM-interacting
‘surfaces’ result from folding in periplasm
(may involve periplasmic Dsb and Ppi enzymes)
OR
• Individual biogenesis pathways – e.g. fimbriae
References
• Navarre and Schneewind. Surface proteins of
Gram-positive bacteria and mechanisms of
their targeting to the cell wall envelope (1999).
Microbiology and Molecular Biology Reviews, 63,
174-229.
• Ton-That et al. Protein sorting to the cell wall
envelope of Gram-positive bacteria (2004).
Biochimica et Biophysica Acta, 1694, 269-278.