Changes in Thylakoid Peptide Composition and Phosphorylation in Differentiated Chloroplasts of Mesophyll and Bundle Sheath Cells of Maize

Abstract

Bundle sheath chloroplasts (BS) of C₄ plants have a low chlorophyll b content, lack the chlorophyll a,b-protein complex, and exhibit low photosystem II activity, while having a high level of photosystem I cyclic electron flow and photophosphorylation. According to the current views on the regulation of intersystem light energy distribution via reversible thylakoid polypeptide phosphorylation, one would expect that if a membrane bound protein kinase were present in BS thylakoids, it should be inactive in vitro, since the plastoquinone pool whose reduction is assumed to activate the kinase, should be mostly in the oxidized state in the absence of electron flow from photosystem II (Haworth et al., 1982). Furthermore, no information is yet available on the polypeptide composition of BS thylakoids. The possibility should be considered that chloroplast differentiation into BS and mesophyll (MS) might be due not only to differential regulation of the nuclear and chloroplast genes coding for the soluble matrix enzyme, RuBP carboxylase, but also for genes coding for membrane-bound polypeptides of photosystem II. In the present work the polypeptide pattern of BS and MS thylakoids of maize and their phosphorylation pattern in vitro was investigated.