Structure of the Mg-chelatase cofactor GUN4 reveals a novel hand-shaped fold for porphyrin binding |
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Authors: | Verdecia Mark A Larkin Robert M Ferrer Jean-Luc Riek Roland Chory Joanne Noel Joseph P |
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Institution: | 1
Chemical Biology and Proteomics Laboratory, Salk Institute for Biological Studies
La Jolla, California
United States of America;2
Howard Hughes Medical Institute, Plant Molecular and Cellular Biology Laboratory
Salk Institute for Biological Studies, La Jolla, California
United States of America;3
The Institut de Biologie Structurale
Grenoble
France;University of North Carolina
United States of America |
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Abstract: | 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-Å 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. |
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