Structure of the Mg-chelatase cofactor GUN4 reveals a novel hand-shaped fold for porphyrin binding.
In plants, the accumulation of the chlorophyll precursor Mg-protoporphyrin IX (Mg-Proto) in the plastid regulates the expression of a number of nuclear genes with functions related to photosynthesis. Analysis of the plastid-to-nucleus signaling activity of Mg-Proto in Arabidopsis thaliana led to the discovery of GUN4, a novel porphyrin-binding protein that also dramatically enhances the activity of Mg-chelatase, the enzyme that synthesizes Mg-Proto. GUN4 may also play a role in both photoprotection and the cellular shuttling of tetrapyrroles. Here we report a 1.78-A resolution crystal structure of Synechocystis GUN4, in which the porphyrin-binding domain adopts a unique three dimensional fold with a "cupped hand" shape. Biophysical and biochemical analyses revealed the specific site of interaction between GUN4 and Mg-Proto and the energetic determinants for the GUN4.Mg-Proto interaction. Our data support a novel protective function for GUN4 in tetrapyrrole trafficking. The combined structural and energetic analyses presented herein form the physical-chemical basis for understanding GUN4 biological activity, including its role in the stimulation of Mg-chelatase activity, as well as in Mg-Proto retrograde signaling.
Verdecia
Mark A
MA
Chemical Biology and Proteomics Laboratory, Salk Institute for Biological Studies, La Jolla, California, USA.
Larkin
Robert M
RM
Ferrer
Jean-Luc
JL
Riek
Roland
R
Chory
Joanne
J
Noel
Joseph P
JP
D017360:Arabidopsis / Q000201:enzymology / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D002734:Chlorophyll / Q000378:metabolism; D003001:Cloning, Molecular; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism*; D008190:Lyases / Q000378:metabolism*; D011166:Porphyrins / Q000378:metabolism*; D017510:Protein Folding; D011994:Recombinant Proteins / Q000378:metabolism; D015183:Restriction Mapping
D029681:Arabidopsis Proteins; C472017:GUN4 protein, Arabidopsis; D047908:Intracellular Signaling Peptides and Proteins; D011166:Porphyrins; D011994:Recombinant Proteins; D002734:Chlorophyll; D008190:Lyases; C086788:magnesium chelatase