Differential Localization of Fraction I Protein between Chloroplast Types

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Abstract
The soluble proteins of C3 and C4 mesophyll chloroplasts and C4 bundle sheath extracts have been analyzed by gel electrophoresis for fraction I protein. Gel scans of soluble protein from C4 bundle sheath extracts and C3 mesophyll chloroplasts showed typical fraction I protein peaks that could be identified by ribulose diphosphate carboxylase activity. No such peak was observed for C4 mesophyll chloroplasts, which also lacked both large and small subunits of ribulose diphosphate carboxylase on sodium dodecyl sulfate gels. The absence of fraction I protein in these chloroplasts was reflected in the soluble protein to chlorophyll ratios, which were roughly 3-fold lower than the ratio obtained for C3 chloroplasts. The carboxylating enzyme in C4 mesophyll cells, phosphoenolpyruvate carboxylase, was found to be a major protein in the cytoplasm of C4 mesophyll protoplasts, and had higher mobility than fraction I protein.