What is helicobacter vac A protein?
vacA is a major virulating factor of h.pylori.the gm -ve
bacterium produces this toxin which after entry into the host cell
causes extensive vacuolation ,resulting cell death.
vac a is a 140kd protein which has a signal seq at its n
terminal end (1-33 amino acid) & an autotransporter domain at c
terminal (1023-1297 amino acids).after entry it breaks into 2
segments -a p33 segment & a p55 segment.these segments form a
channel in the plasma membrane & helps in cell vacuolation
the mechanism of cell vacuolation by vaca is as follows:
The membranes of VacA-induced cell vacuoles contain Rab7 and
other markers for late endocytic compartments
A current model for VacA-induced vacuolation proposes that VacA
is internalized by cells and forms membrane channels in the
membranes of late endocytic compartments .In support of this
model,VacA has been found to
localize to the membranes of VacA-induced vacuoles.Intracellular
expression of VacA in transiently transfected cells results in cell
vacuolation
,which provides additional evidence that it can act at an
intracellular site.An inhibitor of VacA channel formation,
5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), blocks the
ability of VacA to cause cell vacuolation ,and VacA mutant toxins
that lack the ability to form membrane channels also lack the
ability to cause cell vacuolation, regardless of whether they are
applied to the surface of cells or expressed intracellularly in
transiently transfected cells
.Interestingly, a mutant toxin that lacks the ability to form
membrane channels can block the activity of wild-type VacA in a
dominant-negative fashion43
.This dominant-negative phenotype is associated with the
formation of mixed oligomeric structures that comprise both
wild-type and mutant VacA51
.The ability of a dominant-negative mutant toxin to block
wild-type VacA activity provides further evidence that an
oligomeric form of VacA is required for VacA cytotoxicity. Multiple
cellular factors, including vacuolar ATPase,Rab7,Rac1, syntaxin 7
and dynamin, are reported to be required for VacA-induced
vacuolation .These factors might be required for VacA
internalization or the process of vesicle swelling.Overexpression
of PIKfyve kinase has been reported to inhibit VacA-induced
vacuolation,which indicates that VacA-induced cellular alterations
might be related to changes in cellular phosphatidylinositol
metabolism.
An important biophysical question surrounding the process of
VacA-induced vacuole formation concerns the source of the membrane
from which intracellular vacuoles are derived.Massive swelling of
pre-existing vesicular compartments might be expected to result in
lysis of these compartments if an additional source of membrane
were not available.One possibility is that VacA-induced vacuoles
might form as the result of fusion of multiple smaller
endocytic
compartments. In support of this view, the SNARE protein
syntaxin 7,which is involved in intracellular membrane fusion
events, has been localized to the membranes of VacA induced
vacuoles, and intracellular expression of a dominant-negative
mutant form of syntaxin 7 blocks VacA-induced vacuolation
.Another possibility is that vacuoles could arise from late
endosomes without a requirement for fusion of different
compartments via a process involving fusion of late endosomal
internal membranes with the late endosomal limiting
membrane.Further studies are needed to clarify the role of
membrane-fusion events in VacA-induced cell vacuolation.
