Page 6 - Volume 5, Issue 2, July 2012

© Benaki Phytopathological Institute
Skandalis et al. 34
ing dissemination by insect vectors (Triplett
et al. , 2006).
The type VI secretion system
The recently discovered T6SS is typical-
ly encoded by clusters of 13 genes thought
to constitute the minimal set needed to pro-
duce a functional secretion apparatus. Our
current knowledge concerning T6SS reg-
ulation of gene expression and apparatus
components has been obtained basically
from studies on human and animal bacte-
rial pathogens (Boyer et al., 2009). T6SS oc-
cur mostly in α-, β-, and γ-proteobacteria
(about 25% of the sequenced genomes; Bin-
gle et al., 2008) and in Helicobacter hepaticus
(Chow and Mazmanian, 2010), a member of
the ε-(epsilon) group, which is an intestinal
microbiote. Multiple copies of these clusters
(often referred to as T6SS loci in the litera-
ture) are frequently found in the same ge-
nome and are non-orthologous, indicating
that they did not originate from internal du-
plication in the bacterial host but were prob-
ably acquired independently by horizontal
transfer (Sarris et al., 2010). In few such cases
that have been studied, each T6SS assumes
a different role in the interactions of the bac-
terial organism with others.
At present, only few T6SS substrates
have been verified experimentally, but oth-
ers may merely await identification. Some
of the T6SS core component proteins are
structurally related to the cell-puncturing
devices of tailed bacteriophages, and at
least in some well-studied cases, the sys-
tem has been shown capable of translocat-
ing effector proteins into the host cell cyto-
plasm, and thus is implicated in virulence of
certain plant pathogenic bacteria, includ-
ing Agrobacterium tumefaciens, Pectobacte-
rium atrosepticum and Xanthomonas oryz-
ae , as well as in the multi-host pathogen
Pseudomonas aeruginosa strain PA14 (re-
viewed in Sarris et al., 2012). However, the
T6SS is not only implicated in pathogene-
sis, as several reports attribute a fundamen-
tal role in efficient root colonization/nodule
formation by the nitrogen-fixing plant sym-
bionts/mutualists Mesorhizobium loti, Rhizo-
bium leguminosarum and Cupriavidus tai-
wanensis (Bingle et al., 2008).
The diverse role of Bacterial effectors–
not the last words
Bacterial T3SS effectors constitute a large
and diverse group of virulence proteins with
a wide range of cellular-subcellular targets
and biochemical functions. A prominent
feature of these proteins is their modular ar-
chitecture, comprised by domains or mo-
tifs that confer an array of biochemical func-
tions within the eukaryotic cell. They are in
essence molecular chimeras of functional-
ly distinct sequence domains, distributed in
different ways along the effector sequence
properly but with some common rules. For
example, sequence motifs required to direct
secretion from the bacterial cell, transloca-
tion into the host cell and membrane local-
ization, are generally located to the N-termi-
nus of the protein, while functional domains
and motifs mediating subcellular targeting
and interactions with host targets typical-
ly localize to the central or C-terminal por-
tions. This modularity has been exploited
for construction of reporters to monitor se-
cretion/translocation and in genetic screens
designed to identify new effectors. An im-
pressive variety of structural motifs are
found in various combinations in T3SS ef-
fectors (Dean, 2012), which may be viewed
as “genetic jugglery” to borrow a term from
Davis and Davis (2010).
The motifs required for T3SS are locat-
ed in the first 15-25 N-terminal residues,
but without discernible consensus, and a
chaperone binding domain 50 to 150 resi-
dues downstream. Subcellular targeting sig-
nals include mitochondrial targeting motifs,
membrane targetingmotifs nuclear localiza-
tion signals, caspase processing sites and G-
protein regulatory domains, a variety of mo-
tifs involved in protein–protein interactions,
and a long list of catalytic activities (Dean,
2012). These features form the bases of the
diverse cellular roles of effectors in bacteria-
host interactions.
Particular effectors have been found to
control bacterial proliferation on the plant