Differential Localization of Fraction I Protein between Chloroplast Types

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.