Synthesis and Breakdown of the Apoproteins of Light-Harvesting Chlorophyll a/b Proteins in Chlorophyll b-deficient Mutants of Rice

Turnover, in the light, of apoproteins of light-harvesting chlorophyll a/6-proteins for Photo-system I and II (LHC-I and LHC-II, respectively) was studied with the wild-type and three chlorophyll 6-deficient mutants of rice. (1) Synthesis of the 24 and 25 kDa apoproteins of LHC-II and the 20 and 21 kDa apoproteins of LHC-I was examined by incubating leaf segments with [³⁵S]-methionine. The three rice mutants, chlorina 2, which totally lacks chlorophyll b, and chlorina 11 and 14, which are partially deficient in chlorophyll b, synthesized the apoproteins as rapidly as did the wild type rice. (2) Pulse-chase experiments showed that breakdown of the apoproteins proceeded slowly, such that only a small proportion of the newly synthesized apoproteins was lost during the 48 h of the chase in normal rice leaves. By contrast, large fractions of the labelled apoproteins were rapidly degraded within the first several hours of the chase period in the chlorina mutants. The greater the deficiency in chlorophyll b of the mutant, the larger were the rate and extent of the protein breakdown. This result indicates that chlorophyll b is needed to stabilize the apoproteins of LHC-II and LHC-I. (3) However, even in chlorina 2, there were small fractions of the apoproteins with lifetimes as long as those of apoproteins in the wild-type rice, suggesting that the newly synthesized apoproteins are partially protected by a factor(s) other than chlorophyll b. (4) The rate of turnover of the apoproteins was significantly reduced in the dark and strongly inhibited by prior treatment of leaf segments with chloramphenicol.