InterPro domain: IPR037215

GUN4 is a porphyrin-binding protein involved in chlorophyll biosynthesis regulation and intracellular signaling, found in aerobic photosynthetic organisms [ 1 , 2 ]. It has been implicated in retrograde signalling between the chloroplast and nucleus [ 3 ]. GUN4 can bind protoporphyrin IX (PIX) and magnesium protoporphyrin IX (MgPIX), substrate and product of the Mg-chelatase, as well as heme and cobalt protoporphyrin IX (CoPIX) [ 4 ]. It may play a role in protecting plants from reactive oxygen species (ROS)-mediated damage [ 5 ].

GUN4 has a multihelical structure composed of an open array, trapping inside an extended region before the C-terminal helix.

1. Structure of the Mg-chelatase cofactor GUN4 reveals a novel hand-shaped fold for porphyrin binding. PLoS Biol. 3, e151
2. Structure of GUN4 from Chlamydomonas reinhardtii. Acta Crystallogr F Struct Biol Commun 71, 1094-9
3. GUN4-Protoporphyrin IX Is a Singlet Oxygen Generator with Consequences for Plastid Retrograde Signaling. J. Biol. Chem. 291, 8978-84
4. GUN4, a regulator of chlorophyll synthesis and intracellular signaling. Science 299, 902-6
5. GUN4-porphyrin complexes bind the ChlH/GUN5 subunit of Mg-Chelatase and promote chlorophyll biosynthesis in Arabidopsis. Plant Cell 23, 1449-67