Comparative Proteomics of Chloroplast Envelopes from C3 and C4 Plants Reveals Specific Adaptations of the Plastid Envelope to C4 Photosynthesis and Candidate Proteins Required for Maintaining C4 Metabolite Fluxes

Abstract

C4 plants have up to 10-fold higher apparent CO2 assimilation rates than the most productive C3 plants. This requires higher fluxes of metabolic intermediates across the chloroplast envelope membranes of C4 plants in comparison with those of C3 plants. In particular, the fluxes of metabolites involved in the biochemical inorganic carbon pump of C4 plants, such as malate, pyruvate, oxaloacetate, and phosphoenolpyruvate, must be considerably higher in C4 plants because they exceed the apparent rate of photosynthetic CO2 assimilation, whereas they represent relatively minor fluxes in C3 plants. While the enzymatic steps involved in the C4 biochemical inorganic carbon pump have been studied in much detail, little is known about the metabolite transporters in the envelope membranes of C4 chloroplasts. In this study, we used comparative proteomics of chloroplast envelope membranes from the C3 plant pea (Pisum sativum) and mesophyll cell chloroplast envelopes from the C4 plant maize (Zea mays) to analyze the adaptation of the mesophyll cell chloroplast envelope proteome to the requirements of C4 photosynthesis. We show that C3- and C4-type chloroplasts have qualitatively similar but quantitatively very different chloroplast envelope membrane proteomes. In particular, translocators involved in the transport of triosephosphate and phosphoenolpyruvate as well as two outer envelope porins are much more abundant in C4 plants. Several putative transport proteins have been identified that are highly abundant in C4 plants but relatively minor in C3 envelopes. These represent prime candidates for the transport of C4 photosynthetic intermediates, such as pyruvate, oxaloacetate, and malate.