Crystal Structures of GUN4 in Complex with Porphyrins.

GUN4 is a porphyrin-binding protein involved in chlorophyll biosynthesis regulation and intracellular signaling (Larkin et al., 2003). It directly interacts with the catalytic subunit of magnesium chelatase, a branch-point enzyme for the chlorophyll biosynthetic pathway from heme synthesis (Tanaka and Tanaka, 2007). GUN4 can bind to both the substrate and the product of the chelation reaction, protoporphyrin IX (PIX) and magnesium protoporphyrin IX (MgPIX); the latter participates in plastid-to-nucleus retrograde signaling (Nott et al., 2006).