11
Biosynthesis of Bacterial Cellulose
“Guilt by Association” Model of the G. xylinus
Cellulose-Synthesizing Complex
What.is.the.composition.of.a.cellulose-synthesizing.complex.and.how.are.
these.complexes.assembled.and.localized.along.the.longitudinal.axis.in.a.
G. xylinus.cell?.In.the.absence.of.isolated.complexes.and.because.of.a.lack.of.
information.with.respect.to.the.nature.and.number.of.protein.subunits.in.
these.complexes,.any.attempt.to.understand.them.will.require.some.genetic,.
biochemical,. and. structural. information.. Fortunately,. sufficient. information. is. available. from. a. variety. of. studies. to. make. a. beginning. toward.
understanding.the.possible.assembly.of.a.cellulose-synthesizing.complex..
Although. protein–protein. interactions. have. not. been. demonstrated. for.
any.of.the.proteins.coded.for.by.genes.in.the.bcs.operon,.the.fact.that.they.
are. coded. in. this. operon. makes. them. possible. candidates. for. interaction.
(guilt by.association.in.an.operon)..In.addition,.mutant.analysis.has.clearly.
shown.that.all.the.proteins.coded.by.genes.in.the.bcs.operon.have.a.role.in.
cellulose. . synthesis.. At. a. minimum,. each. cellulose-synthesizing. site. (a. TC.
subunit).could.be.comprised.of.a.homomultimeric.assembly.of.the.cellulose.
synthase.catalytic.subunits.(BcsAs)..Considering.the.hierarchical.assembly.
of. sheets. to. minicrystals. to. microfibrils. to. a. ribbon. in. G. xylinus. (Brown.
1996),. each. TC. subunit. could. be. an. assembly. of. up. to. 16. closely. packed.
BcsAs.in.the.cytoplasmic.membrane..Since.bcsB.is.present.in.almost.all.bacteria.next.to.the.bcsA.coding.region.or.as.part.of.the.bcsAB.gene,.BcsB.must.
be.part.of.the.cellulose.synthase.complex,.most.likely.in.equal.proportions.
to.BcsA..Biochemical.analysis.of.purified.cellulose.synthase.always.shows.
the.presence.of.an.83.kD.polypeptide.(corresponding.to.BcsA).and.a.93.kD.
polypeptide.(corresponding.to.BcsB),.suggesting.a.tight.association.of.these.
two.polypeptides.in.a.1:1.ratio.(Chen.and.Brown.1996)..Sequence.analysis.
and. TnPhoA. mutagenesis. experiments. suggest. that. BcsB. present. in. the.
cytoplasmic.membrane.as.a.transmembrane.protein.is.exposed.much.more.
on.the.periplasmic.side.of.the.membrane.(Saxena.et.al..1994)..The.BcsC.protein.is.present.in.the.outer.membrane.through.a.β-barrel.fold.and.extends.
into. the. periplasmic. space. where. it. has. a. number. of. TPR. repeats. that. are.
suggested. to. function. in. protein–protein. interactions. (Keiski. et. al.. 2010)..
The.periplasmic.region.of.BcsC.most.likely.associates.with.the.periplasmic.
portion.of.BcsB..The.BcsD.protein.is.most.likely.a.cytosolic.protein,.and.it.
forms.a.complex.that.associates.with.the.cytosolic.region.of.BcsA..The.glucan.chains.synthesized.in.the.cytosolic.region.of.BcsA.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.(Figure 1.2).