Proteomic Analysis of the Proplastid Envelope Membrane Provides Novel Insights into Small Molecule and Protein Transport across Proplastid Membranes

Proplastids are undifferentiated plastids of meristematic tissues that synthesize amino acids for protein synthesis, fatty acids for membrane lipid production, and purines and pyrimidines for DNA and RNA synthesis. Unlike chloroplasts, proplastids depend on supply, with reducing power, energy, and precursor metabolites from the remainder of the cell. Comparing proplastid and chloroplast envelope proteomes and the corresponding transcriptomes of leaves and shoot apex revealed a clearly distinct composition of the proplastid envelope. It is geared towards import of metabolic precursors and export of product metabolites for the rapidly dividing cell. The analysis also suggested a new role for the triosephosphate translocator in meristematic tissues, identified the route of organic nitrogen import into proplastids, and detected an adenine nucleotide exporter. The protein import complex contains the import receptors Toc120 and Toc132 and lacks the redox sensing complex subunits of Tic32, Tic55, and Tic62, which mirrors the expression patterns of the corresponding genes in leaves and the shoot apex. We further show that the protein composition of the internal membrane system is similar to etioplasts, as it is dominated by the ATP synthase complex and thus remarkably differs from that of chloroplast thylakoids.