Identification and differential accumulation of two isoforms of the CF1-β subunit under high light stress in Brassica rapa

The chloroplast ATP synthase coupling factor CF1 complex contains five nonidentical subunits, α, β, γ, δ, and ε, with a stoichiometry of 3:3:1:1:1. The β subunit contains the catalytic site for ATP synthesis during photooxidative phosphorylation in the chloroplast. In this study, we have identified two isoforms of the CF1-β subunit at 56 and 54 kDa in the chloroplast of Brassica rapa, through isolation/purification, proteolytic digestion and internal peptide sequencing. Examining their accumulation pattern demonstrates that both isoforms coexist during chloroplast biogenesis and in mature thylakoid membranes, but the 54 kDa isoform is more apparently upregulated by light or under light stress. LiDS-PAGE shows that the 56 kDa is a major isoform of the CF1-β subunit under normal light conditions, and its amount was not influenced during high light or other light stress treatments. The 54 kDa isoform is a minor band at normal conditions; however, it significantly increased under excess light stresses, such as high or low light with drought and/or high temperature. Particularly, a ninefold increase was observed after 8–10 h of high light treatment with drought and high temperature. The results suggest that light stress induction of the 54 kDa CF1-β isoform may present a positive response during chloroplast photoacclimation.