10
Bacterial Cellulose
The. catalytic. subunit. of. cellulose. synthase. coded. by. bcsA. or. bcsAB. is. a.
processive.β-glycosyltransferase.(Saxena.et.al..1995).that.belongs.to.glycosyltransferase.family.2.(GT-2)..It.is.the.only.protein.coded.for.by.a.gene.in.the.bcs.
operon.for.which.a.defined.role.in.cellulose.biosynthesis.is.known..This.protein.catalyzes.the.synthesis.of.β-1,4-linked.glucan.chains.using.UDP-glucose.
as.a.substrate,.and.it.is.activated.by.c-di-GMP..It.is.a.membrane.protein.that.
is. localized. in. the. cytoplasmic. membrane.. It. has. multiple. transmembrane.
segments.and.a.large.globular.region.that.is.present.in.the.cytosol..The.cytosolic.globular.region.contains.the.conserved.sequence.motif.D,D,D,QXXRW..
Residues. in. the. conserved. motif. function. in. the. catalytic. activity.. The.
structure.of.this.protein.has.not.been.determined,.although.from.sequence.
analysis.it.has.been.suggested.to.have.the.GT-A.fold.found.in.many.glycosyltransferases.. This. protein. contains. the. c-di-GMP-binding. PilZ. domain. for.
activation.by.c-di-GMP.(Ryjenkov.et.al..2006).
The.BcsB.protein.is.coded.by.bcsB.or.bcsAB..It.is.a.transmembrane.protein,. with. most. of. the. protein. being. exposed. to. the. periplasmic. space..
This. protein. was. originally. thought. to. be. the. c-di-GMP-binding. subunit.
of.the.cellulose.synthase;.however,.identification.of.the.c-di-GMP-binding.
PilZ. domain. in. BcsA. suggests. that. BcsB. is. not. the. c-di-GMP. component.
of.. cellulose.synthase..Using.antibodies,.this.protein.has.been.shown.to.be.
localized. to. the. linear. terminal. complexes. along. the. longitudinal. axis. of.
the.cell.(Kimura.et.al..2001)..This.protein.does.not.show.significant.homology.to.any.other.protein.in.the.database..The.bcsB.gene.has.been.found.to.
be.present.with.bcsA.in.the.bcs.operon.of.almost.all.bacteria.that.carry.the.
cellulose.synthase.gene.
BcsC. is. predicted. to. be. localized. to. the. outer. membrane. and. the. periplasmic. space.. Using. modeling. to. determine. the. structure. of. this. protein,.
it. is. predicted. to. fold. into. a. β-barrel. structure. that. is. present. in. the. outer.
membrane. and. a. region. with. tetratricopeptide. repeat. (TPR). motifs. that. is.
. present.in.the.periplasmic.space..Bioinformatics.analysis.suggests.that.BcsC.
is.involved.in.the.export.of.cellulose.(Keiski.et.al..2010)..This.protein.is.not.
required.for.in.vitro.cellulose.synthase.activity,.but.it.is.required.for.cellulose.production.in.vivo..The.gene.for.this.protein.is.present.in.the.bcs.operon.
of.almost.all.bacteria.that.have.this.operon.
BcsD. is. a. soluble. protein. coded. by. bcsD. in. the. bcs. operon.. The. structure.
of. BcsD. has. been. determined. (Hu. et. al.. 2010).. The. protein. assembles. into.
a. . cylinder-shaped. structure. with. eight. subunits. and. four. inner. passageways. for. glucan. chains.. The. cellular. localization. of. this. protein. is. not. confirmed,.although.it.has.been.suggested.to.be.present.in.the.periplasmic.space.
(Iyer et al..2011)..This.protein.is.not.required.for.in.vitro.cellulose.synthase.
activity. and. is. not. essential. for. cellulose. production. in. vivo. bcsD. mutants.
produce.small.amounts.of.cellulose..The.bcsD.gene.is.found.in.G. xylinus.and.
only.a.few.other.bacterial.species.
Bacterial Cellulose
The. catalytic. subunit. of. cellulose. synthase. coded. by. bcsA. or. bcsAB. is. a.
processive.β-glycosyltransferase.(Saxena.et.al..1995).that.belongs.to.glycosyltransferase.family.2.(GT-2)..It.is.the.only.protein.coded.for.by.a.gene.in.the.bcs.
operon.for.which.a.defined.role.in.cellulose.biosynthesis.is.known..This.protein.catalyzes.the.synthesis.of.β-1,4-linked.glucan.chains.using.UDP-glucose.
as.a.substrate,.and.it.is.activated.by.c-di-GMP..It.is.a.membrane.protein.that.
is. localized. in. the. cytoplasmic. membrane.. It. has. multiple. transmembrane.
segments.and.a.large.globular.region.that.is.present.in.the.cytosol..The.cytosolic.globular.region.contains.the.conserved.sequence.motif.D,D,D,QXXRW..
Residues. in. the. conserved. motif. function. in. the. catalytic. activity.. The.
structure.of.this.protein.has.not.been.determined,.although.from.sequence.
analysis.it.has.been.suggested.to.have.the.GT-A.fold.found.in.many.glycosyltransferases.. This. protein. contains. the. c-di-GMP-binding. PilZ. domain. for.
activation.by.c-di-GMP.(Ryjenkov.et.al..2006).
The.BcsB.protein.is.coded.by.bcsB.or.bcsAB..It.is.a.transmembrane.protein,. with. most. of. the. protein. being. exposed. to. the. periplasmic. space..
This. protein. was. originally. thought. to. be. the. c-di-GMP-binding. subunit.
of.the.cellulose.synthase;.however,.identification.of.the.c-di-GMP-binding.
PilZ. domain. in. BcsA. suggests. that. BcsB. is. not. the. c-di-GMP. component.
of.. cellulose.synthase..Using.antibodies,.this.protein.has.been.shown.to.be.
localized. to. the. linear. terminal. complexes. along. the. longitudinal. axis. of.
the.cell.(Kimura.et.al..2001)..This.protein.does.not.show.significant.homology.to.any.other.protein.in.the.database..The.bcsB.gene.has.been.found.to.
be.present.with.bcsA.in.the.bcs.operon.of.almost.all.bacteria.that.carry.the.
cellulose.synthase.gene.
BcsC. is. predicted. to. be. localized. to. the. outer. membrane. and. the. periplasmic. space.. Using. modeling. to. determine. the. structure. of. this. protein,.
it. is. predicted. to. fold. into. a. β-barrel. structure. that. is. present. in. the. outer.
membrane. and. a. region. with. tetratricopeptide. repeat. (TPR). motifs. that. is.
. present.in.the.periplasmic.space..Bioinformatics.analysis.suggests.that.BcsC.
is.involved.in.the.export.of.cellulose.(Keiski.et.al..2010)..This.protein.is.not.
required.for.in.vitro.cellulose.synthase.activity,.but.it.is.required.for.cellulose.production.in.vivo..The.gene.for.this.protein.is.present.in.the.bcs.operon.
of.almost.all.bacteria.that.have.this.operon.
BcsD. is. a. soluble. protein. coded. by. bcsD. in. the. bcs. operon.. The. structure.
of. BcsD. has. been. determined. (Hu. et. al.. 2010).. The. protein. assembles. into.
a. . cylinder-shaped. structure. with. eight. subunits. and. four. inner. passageways. for. glucan. chains.. The. cellular. localization. of. this. protein. is. not. confirmed,.although.it.has.been.suggested.to.be.present.in.the.periplasmic.space.
(Iyer et al..2011)..This.protein.is.not.required.for.in.vitro.cellulose.synthase.
activity. and. is. not. essential. for. cellulose. production. in. vivo. bcsD. mutants.
produce.small.amounts.of.cellulose..The.bcsD.gene.is.found.in.G. xylinus.and.
only.a.few.other.bacterial.species.