Bacterial protein secretion systems
Many proteins need to be
transported across the
bacterial membrane
These include flagella and
pilus subunits
Gram negative bacteria have
evolved several systems for
the secretion of proteins to
the external environment
Bacterial protein secretion systems
Different systems are named
according to the order in
which they were discovered
i.e. Type I, II, III, IV, etc.
Gram negative protein secretion systems
Gram negative protein secretion systems
Sec-dependent pathway
Also known as the general
secretory pathway (GSP)
Most commonly used
system to transport proteins
across plasma membrane or
integrate them into the
membrane
Used by both gram positive
and gram negative bacteria
Sec-dependent pathway
System composed of both
cytosolic and membranebound proteins
Secreted proteins contain Nterminal signal sequence
Chaperone protein binds to
sequence and guides protein
to transport machinery or
translocon
Sec-dependent pathway
Chaperones include SecB
and signal recognition
particle (SRP)
Chaperones release
preprotein which binds to
SecA
Sec-dependent pathway
SecA translocates preprotein
through SecYEG complex
Hydrolysis of ATP drives
translocation
After emerging from the
membrane, signal peptide is
removed by signal peptidase
Type II secretion systems
Found in many gramnegative bacteria, including
species of Erwinia,
Pseudomonas and Vibrio
Used for the secretion of
proteins through the outer
membrane
Composed of as many as 14
different proteins
(both inner and outer
membrane-localized)
Type II secretion systems
Outer membrane protein
forms a channel through
which protein can pass
Family of proteins referred
to as secretins
Secretins
Type I secretion systems
Also known as ABC
protein secretion pathway
ABC = ATP-binding cassette
Secretion is independent of
the sec pathway
Type I secretion systems
Used for the secretion of
various enzymes and toxins
including -hemolysin
Secreted proteins contain a
C-terminal (noncleavable)
signal sequence
Type I secretion systems
Are composed of only three
different proteins
1. ATP -binding inner
membrane protein
2. Outer membrane
channel-forming protein
3. Membrane fusion
protein
Type I secretion systems
Type I secretion systems
Proteins transported across
both membranes in a single
step
Energy for translocation
derived from hydrolysis of
ATP
Type III secretion systems
Found in a number of gramnegative bacteria
Bacteria with TTSS always
exhibit intimate contact with
host cells
Systems designed to both
secrete and translocate or
“inject” proteins into host
cells
Type III secretion systems
Composed of at least 20
different proteins
Include cytosolic,
membrane-bound and
secreted proteins
Membrane proteins form a
structure similar to the
flagella basal body named
the needle complex
Type III secretion systems
Outer membrane component
of needle complex belongs
to the secretin family of
proteins
Proteins transported across
both membranes in a single
step
Hydrolysis of ATP provides
energy for translocation
Type III secretion systems
Type III secretion systems
Type III secretion systems
Type III secretion systems
Many protein required for
secretion through needle
complex are similar to
flagella assembly proteins
TTSSs likely evolved from
bacterial flagella systems
Type IV secretion systems
Can be used to transfer
proteins or DNA from one
cell to another
Transfer proteins to
eukaryotic host cells
Transfer plasmids to
bacterial cells (conjugation)
Type IV secretion systems
Composed of several
proteins that span the entire
cell wall
Hydrolysis of ATP provides
energy for transport
Found in many bacterial
pathogens including species
of Legionella, Bordetella
and Helicobacter
Conjugal pilus
Several morphologically
distinct pili
Appear to function as
adhesins rather than transfer
conduits
Retraction of pilus pulls
cells close together
Type IV secretion/ autotransporters
Transport through inner
membrane is sec-dependant
All substrates carry secsignal sequence
Proteins divided into three
domains i) sec signal
sequence, ii) passenger
domain, iii) -barrel domain
Type IV secretion/ autotransporters
Type IV secretion/ autotransporters
OM
ATP
Sec
Y
E
G
SecA
ADP
Sec
Y
E
G
IM
N
SecB
C
Gram negative protein secretion systems