Molecular basis for semidominance of missense mutations in the XANTHA-H (42-kDa) subunit of magnesium chelatase

During biosynthesis of bacteriochlorophyll or chlorophyll, three protein su bunits of 140, 70, and 42 kDa interact to insert Mg2+ into protoporphyrin I X. The semidominant Chlorina-125, -157, and -161 mutants in barley are defi cient in this step and accumulate protoporphyrin IX after feeding on 5-amin olevulinate. Chlorina-125, -157, and -161 are allelic to the recessive xant ha-h mutants and contain G559A, G806A, and C271T mutations, respectively. T hese mutations cause single amino acid substitutions in residues that are c onserved in all known primary structures of the 42-kDa subunit, In vitro co mplementation and reconstitution of Mg-chelatase activity show that the 42- kDa subunits are defective in the semidominant Chlorina mutants. A mutated protein is maintained in the Chlorina plastids, unlike in the xantha-h plas tids. Heterozygous Chlorina seedlings have 25-50% of the the Mg-chelatase a ctivity of wild-type seedlings, Codominant expression of active and inactiv e 42-kDa subunits in heterozygous Chlorina seedlings is likely to produce t wo types of heterodimers between the strongly interacting 42-kDa and 70-kDa subunits. Reduced Mg-chelatase activity is explained by the capacity of he terodimers consisting of mutated 42-kDa and wild-type 70-kDa protein to bin d to the 140-kDa subunit. The 42-kDa subunit is similar to chaperones that refold denatured polypeptides with respect to its ATP-to-ADP exchange activ ity and its ability to generate ATPase activity with the 70-kDa subunit. We hypothesize that the association of the 42-kDa subunit with the 70-kDa sub unit allows them to form a specific complex with the 140-kDa subunit and th at this complex inserts Mg2+ into protoporphyrin IX.