Abstract
Mg-chelatase composed by CHLI, CHLD, and CHLH subunits catalyzes the insertion of Mg2+ into protoporphyrin IX, the first step of chlorophyll biosynthesis. CHLH is catalytic subunit that binds substrate or product porphyrin. Recently, CHLH is proposed to be a plastid-localized abscisic acid receptor, and furthermore, mutations in CHLH resulted in gun phenotype that defects in plastid to nuclear signaling. We have previously reported in vitro analysis of porphyrin affinities of mutant CHLH proteins showing gun phenotype. Here, we found CHLH-porphyrin complexes were rapidly degraded upon illumination in vitro. Consistent with dissociation constants of CHLH to porphyrins, CHLH-protoporphyrin IX complex was more sensitive to light degradation, when compared to that of CHLH-Mg-protoporphyrin IX complex that has lower affinity to CHLH. Interestingly, in vivo CHLH protein level of a gun mutant, cch, was much higher that that of wild type, although the gene expression levels were comparable each other. It is therefore likely that binding of porphyrin to CHLH involves post-translational light degradation of CHLH. Physiological significance of the CHLH photo-degradation will be discussed.
