一光敏色素蛋白结构被解析

佚名

For  plants,  the  ability  to  accurately  sense  light  governs  everything  from  seed  germination,  photosynthesis  and  pigmentation  to  patterns  of  growth  and  flowering.

Now,  for  the  first  time,  scientists  have  obtained  a  detailed  map  of  one  of  biology's  most  important  light  detectors,  a  protein  found  in  many  species  across  life's  plant,  fungal,  and  bacterial  kingdoms.  By  resolving  the  three-dimensional  structure  of  the  protein  known  as  phytochrome,  scientists  can  now  tease  out  the  secrets  of  how  plants,  in  particular,  react  to  light,  opening  the  door  for  a  host  of  manipulations  that  could  have  sweeping  implications  for  agriculture.

Writing  in  the  Nov.  17  issue  of  the  journal  Nature,  a  team  of  scientists  from  UW-Madison  report  that  they  have  obtained  the  crystal  structure  of  a  phytochrome  from  a  bacterium,  the  first  such  light-gathering  structure  depicted  for  all  of  biology.  The  structure  of  the  bacterial  phytochrome,  according  to  the  report,  suggests  its  architecture  first  arose  a  billion  or  so  years  ago  in  a  common  ancestor  and  is  shared  among  not  only  bacteria,  but  also  by  plants  and  fungi.

"This  is  probably  the  most  important  light  regulator  in  agriculture,"  says  Richard  Vierstra,  a  UW-Madison  plant  geneticist  and  one  of  two  collaborating  senior  authors  of  the  Nature  paper.  "It  tells  plants  when  to  germinate.  It  tells  them  where  to  grow  to  absorb  the  most  light  and  to  avoid  competition.  It  tells  them  when  to  flower.  It  tells  them  when  to  die  at  the  end  of  the  growing  season."

The  accomplishment  of  the  Wisconsin  researchers,  including  first  author  graduate  student  Jeremiah  Wagner,  caps  a  30-year  quest  by  biologists  to  drill  down  to  the  inner  workings  of  how  plants,  fungi  and  bacteria  use  light  to  guide  their  development.  It  will  likely  spur  a  rush  by  scientists  to  capitalize  on  the  new  knowledge  and  may  one  day  lead  to  such  things  as  plants  whose  growth,  flowering  and  death  can  be  precisely  manipulated.

"We  can  now  start  changing  how  phytochromes  work  in  a  rational  way  to  improve  how  plants  respond  to  light,"  says  Wagner.  "People  have  been  trying  to  do  this  for  a  long  time.  Practically  speaking,  we  can  now  try  to  re-engineer  the  vision  system  of  a  plant."

According  to  Vierstra,  there  are  many  kinds  of  phytochromes  found  in  every  plant,  and  they  exist  in  virtually  all  cells.  They  occur  in  greater  concentrations  in  cells  that  respond  directly  to  light,  such  as  in  root  tips  and  new  shoots.

The  phytochrome  revealed  by  the  Wisconsin  team  was  derived  from  a  microbe  known  as  Deinococcus  radiodurans,  a  bacterium  renowned  for  its  tolerance  to  ionizing  radiation.  It  was  only  within  the  last  eight  years  that  scientists  from  Vierstra's  and  other  labs  discovered  that,  like  plants,  some  bacteria  harbor  phytochromes.  That  finding  opened  the  way  for  the  Wisconsin  team  to  define  the  structure  of  a  phytochrome,  as  bacteria  are  easy  to  grow  in  the  lab  and  their  proteins  are  easier  to  purify  and  manipulate  than  plant  proteins.

Once  isolated,  the  phytochrome  was  crystallized  and  its  molecular  structure  was  mapped  using  a  beam  of  X-rays  to  develop  a  three-dimensional  picture  of  the  protein.  That  three-dimensional  portrait,  which  reveals  the  atom-by-atom  configuration  of  the  molecule,  is  the  key  to  understanding  the  "nuts  and  bolts"  of  how  the  photoreceptor  senses  light  and  triggers  a  series  of  downstream  events  that  control  growth  and  development,  according  to  Katrina  Forest,  the  other  senior  member  of  the  team  and  a  UW-Madison  professor  of  bacteriology.

"There  were  some  surprises,"  says  Forest  of  the  ribbon-like  protein.  "This  protein  has  a  knot."  That  is  a  startling  feature,  she  notes,  observed  in  only  a  handful  of  proteins  out  of  tens  of  thousands  whose  structures  are  known.  The  group  speculates  the  knot  may  help  stabilize  the  protein  so  it  can  do  its  job  of  capturing  light  and  triggering  the  cascade  of  downstream  events  under  its  control.

"We  think  that  without  the  knot,  the  protein  conformational  changes  (prompted  by  light)  would  be  too  floppy  to  be  efficiently  channeled  to  downstream  proteins,"  Forest  explains.

Phytochromes  have  unique  properties  that  enable  them  to  switch  between  two  stable  states  that  sense  red  and  far-red  light.  The  light  is  actually  detected  by  a  specialized  pigment  that  sits  within  a  pocket  on  the  protein.  Red  and  far-red  light,  says  Forest,  have  the  effect  of  reversibly  changing  the  structure  of  the  pigment  in  the  pocket,  which  then  flips  a  switch  on  the  protein  to  trigger  the  events  of  growth  and  development.

Intriguingly,  the  phytochrome  has  the  ability  to  store  the  light  it  has  detected,  initiating  a  response  days  after  it  is  sensed,  Vierstra  says.  "This  memory  allows  the  plant  to  predict  where  the  light  will  come  from  each  day  and  measure  the  length  of  daylight  so  that  they  flower  in  the  correct  season."

By  deducing  the  architecture  of  the  phytochrome  protein,  according  to  Vierstra  and  Forest,  it  may  be  possible  to  engineer  and  introduce  into  crops  phytochromes  that  respond  to  different  wavelengths  of  light,  or  are  more  or  less  active.  These  changes,  in  turn,  could  allow  plants  to  grow  under  different  climate  regimes  or  flower  at  different  times  of  the  year,  for  example.

Gaining  precise  control  over  flowering  events,  says  Vierstra,  is  a  key  to  the  success  or  failure  of  most  crops,  as  most  of  what  we  eat  comes  from  seeds  and  fruits  produced  by  flowers.  In  another  scenario,  it  may  be  possible  to  dampen  the  role  of  phytochromes  in  crop  plants  to  avoid  having  them  compete  with  each  other  for  light  when  grown  close  together  in  a  field.

In  addition  to  Wagner,  Vierstra  and  Forest,  the  paper  was  authored  by  Joseph  S.  Brunzelle  of  Northwestern  University.  The  work  was  funded  primarily  by  the  National  Science  Foundation.  Funding  was  also  provided  by  the  U.S.  Department  of  Energy  and  the  W.M.  Keck  Foundation.