Handling Copper: A Paradigm for the Mechanisms of Metal Ion Homeostasis - Environmental Contamination: Health Risks and Ecological Restoration - page 286

sequence.. Although. paa1 . mutants. show. a. decrease. in. growth. rate. due. to.
impairment. of. photosynthetic.activity,.it. is. not.a. lethal. mutation.(Shikanai.
et.al..2003)..As.a.result.of.this.phenotype,.it.is.thought.that.copper.can.enter.
the. chloroplast. membrane. through. an. alternate. route.. Recently,. another.
P-type. ATPase,. HMA1,. has. been. localized. to. the. chloroplastic. membrane.
and.it.has.been.implicated.as.an.alternate.copper.transporter.for.SOD.activ-
ity.(Seigneurin-Berny.et.al..2006).
In. addition. to. the. Cu,. Zn. SOD. in. the. stroma,. the. chloroplast. has. a. sec-
ond. more. important. target. for. copper,. plastocyanin.. Under. varying. cop-
per.conditions,.the.plant.must.balance.regulation.of.copper.to.both.of.these.
targets.within.the.chloroplast..Obviously,.photosynthetic.activity.will.have.
a. higher. priority. for. acquiring. copper,. but. the. regulators. for. this. homeo-
static.mechanism.are.unknown..Through.GFP.analysis,.the.CCS.protein.has.
been. localized. to. the. stroma. (Abdel-Ghany. et. al.. 2005a). and. additionally.
two.other.putative.copper.chaperones.have.been.localized.to.the.chloroplast.
(Burkhead. et. al.. 2003,. Burkhead. and. Colorado. State. University. 2003).. The.
putative. chaperone,. CCP. (Copper. Chaperone. for. the. Plastid),. is. somewhat.
similar. to. the. bacterial. CopZ. and. ATX1. proteins. in. sequence. and. has. an.
ATX-like.βαββαβ.fold..However,.CCP.does.not.contain.a.typical.metal.bind-
ing. domain. (MxCxxC);. instead. it. has. a. series. of. serine. repeats. (Burkhead.
and. Colorado. State. University. 2003).. The. function. of. this. protein. is. cur-
rently.unknown;.however,.one.hypothesis.is.that.it.could.be.the.chaperone.
for.plastocyanin.(Burkhead.and.Colorado.State.University.2003)..The.second.
putative. copper. chaperone,. CutA,. also. does. not. have. a. stereotypical. metal.
binding.site;.however,.there.is.sequence.similarity. to.a.copper-related.pro-
tein.in. E. coli .by.the.same.name..This.protein.is.expressed.in.all.major.plant.
tissues.at.similar.levels.and.puriied.recombinant.protein.of.the. Arabidopsis .
CutA.has.been.shown.to.bind.Cu(II).at.a.level.of.nearly.one.mole.copper.per.
one.mole.of.protein.(Burkhead.et.al..2003).
The. chloroplast. is. a. complex. organelle. and. has. several. membranes. for.
copper.to.transverse.across..Even.though.two.of.the.ive.target.proteins.for.
copper.are.located.in.the.chloroplast,.transport.to.the.organelle.and.within.
are.still.not.well.understood..No.copper. chaperone.has.been.identiied.for.
transport.to.PAA1.in.the.chloroplast.membrane.or.to.PAA2.in.the.thylakoid.
membrane.for.plastocyanin.
References
Abdel-Ghany.SE,.Burkhead.JL,.Gogolin.KA.et.al..2005a..AtCCS.is.a.functional.homo-
log.of.the.yeast.copper.chaperone.Ccs1/Lys7.. FEBS Letters .579:.2307-2312.
Abdel-Ghany.SE,.Muller-Moule.P,.Niyogi.KK,.Pilon.M,.Shikanai.T..2005b..Two.P-type.
ATPases. are. required. for. copper. delivery. in. Arabidopsis thaliana . chloroplasts..
Plant Cell .17:.1233-1251.