Figure 1.2
12
Bacterial Cellulose
Top view
Side view
β1,4glucan chain
BcsC– βbarrel
BcsC TPR
BcsB
BcsA
BcsD
Extracellular space
Outer membrane
Periplasmic space
Cytoplasmic membrane
Cytosol
Figure 1.2
Organization.of.proteins.in.a.cellulose-synthesizing.complex.(TC.subunit).in.G. xylinus..The.
top.view.shows.the.cytosolic.side.of.16.cellulose.synthase.catalytic.subunits.(BcsA).that.are.
suggested.to.be.present.in.a.single.complex.in.the.cytoplasmic.membrane..The.side.view.shows.
four.β-1,4-glucan.chains.being.synthesized.and.translocated.across.the.bacterial.envelope.in.
a.region.with.an.assembly.of.four.BcsA.molecules.in.association.with.BcsB,.BcsC,.and.BcsD..
Although.BcsA.is.localized.in.the.cytoplasmic.membrane,.a.large.globular.region.of.this.protein. is. present. in. the. cytosol,. and. it. is. this. region. that. contains. the. catalytic. site.. The. BcsB.
protein.is.also.localized.in.the.cytoplasmic.membrane,.but.it.is.exposed.more.toward.the.periplasmic. space.. This. protein. interacts. with. BcsA. in. the. transmembrane. region. and. with. the.
TPR-containing.region.of.BcsC.in.the.periplasmic.space..BcsC.is.present.in.the.outer.membrane.
where.it.has.a.β-barrel.fold..It.also.extends.into.the.periplasmic.space.and.this.region.of.BcsC.
has.a.number.of.TPR.repeats.that.allow.protein–protein.interactions..The.localization.of.BcsD.
is.still.uncertain,.although.there.has.been.a.suggestion.of.it.being.present.in.the.periplasmic.
space..In.the.absence.of.any.sequence.features.for.it.to.be.transported,.we.feel.that.this.protein.
is.localized.in.the.cytosol.where.it.may.interact.with.the.cytosolic.region.of.BcsA.molecules.
and.keep.them.in.an.assembly..BcsD.may.also.interact.with.glucan.chains.as.they.move.from.
the. cytosol. into. the. BcsA. membrane. region.. The. glucan. chains. synthesized. in. the. cytosolic.
region.of.BcsA.are.shown.to.stay.closely.attached.to.the.BcsA-BcsB-BcsC.complex.in.a.rigid.and.
unassembled.state.until.they.reach.the.extracellular.side,.where.they.can.associate.with.each.
other.to.form.higher-order.structures.
Many Unanswered Questions Remain
The. synthesis. of. a. polysaccharide. chain. can. be. visualized. as. taking. place.
in. three. stages:. initiation,. elongation,. and. termination,. much. like. the. synthesis.of.nucleic.acids.and.proteins..However,.unlike.the.requirement.of.a.
template.for.the.synthesis.of.these.polymers,.no.template.is.required.for.the.
12
Bacterial Cellulose
Top view
Side view
β1,4glucan chain
BcsC– βbarrel
BcsC TPR
BcsB
BcsA
BcsD
Extracellular space
Outer membrane
Periplasmic space
Cytoplasmic membrane
Cytosol
Figure 1.2
Organization.of.proteins.in.a.cellulose-synthesizing.complex.(TC.subunit).in.G. xylinus..The.
top.view.shows.the.cytosolic.side.of.16.cellulose.synthase.catalytic.subunits.(BcsA).that.are.
suggested.to.be.present.in.a.single.complex.in.the.cytoplasmic.membrane..The.side.view.shows.
four.β-1,4-glucan.chains.being.synthesized.and.translocated.across.the.bacterial.envelope.in.
a.region.with.an.assembly.of.four.BcsA.molecules.in.association.with.BcsB,.BcsC,.and.BcsD..
Although.BcsA.is.localized.in.the.cytoplasmic.membrane,.a.large.globular.region.of.this.protein. is. present. in. the. cytosol,. and. it. is. this. region. that. contains. the. catalytic. site.. The. BcsB.
protein.is.also.localized.in.the.cytoplasmic.membrane,.but.it.is.exposed.more.toward.the.periplasmic. space.. This. protein. interacts. with. BcsA. in. the. transmembrane. region. and. with. the.
TPR-containing.region.of.BcsC.in.the.periplasmic.space..BcsC.is.present.in.the.outer.membrane.
where.it.has.a.β-barrel.fold..It.also.extends.into.the.periplasmic.space.and.this.region.of.BcsC.
has.a.number.of.TPR.repeats.that.allow.protein–protein.interactions..The.localization.of.BcsD.
is.still.uncertain,.although.there.has.been.a.suggestion.of.it.being.present.in.the.periplasmic.
space..In.the.absence.of.any.sequence.features.for.it.to.be.transported,.we.feel.that.this.protein.
is.localized.in.the.cytosol.where.it.may.interact.with.the.cytosolic.region.of.BcsA.molecules.
and.keep.them.in.an.assembly..BcsD.may.also.interact.with.glucan.chains.as.they.move.from.
the. cytosol. into. the. BcsA. membrane. region.. The. glucan. chains. synthesized. in. the. cytosolic.
region.of.BcsA.are.shown.to.stay.closely.attached.to.the.BcsA-BcsB-BcsC.complex.in.a.rigid.and.
unassembled.state.until.they.reach.the.extracellular.side,.where.they.can.associate.with.each.
other.to.form.higher-order.structures.
Many Unanswered Questions Remain
The. synthesis. of. a. polysaccharide. chain. can. be. visualized. as. taking. place.
in. three. stages:. initiation,. elongation,. and. termination,. much. like. the. synthesis.of.nucleic.acids.and.proteins..However,.unlike.the.requirement.of.a.
template.for.the.synthesis.of.these.polymers,.no.template.is.required.for.the.