SOLUTE TRANSPORTERS OF THE PLASTID ENVELOPE MEMBRANE

Plastidssemi-autonomous, membrane-bound organelles of plant cells that carry out a large number of biosynthetic and other functions such as photosynthesis (chloroplasts), ammonia and sulfur assimilation, and starch storage (amyloplasts) are metabolically extraordinarily active and versatile organelles that are found in all plant cells with the exception of angiosperm pollen grains. Many of the plastid-localized biochemical pathways depend on precursors from the cytosol and, in turn, many cytosolic pathways depend on the supply of precursor molecules from the plastid stroma. Hence, a massive traffic of metabolites occurs across the permeability barrier between plastids and cytosol that is called the plastid envelope membrane. Many of the known plastid envelope solute transportersintegral membrane proteins that catalyze the transport of solutes across biological lipid-bilayer membranes have been identified by biochemical purification and peptide sequencing. This approach is of limited use for less abundant proteins and for proteins of plastid subtypes that are difficult to isolate in preparative amounts. Hence, the majority of plastid envelope membrane transporters are not yet identified at the molecular level. The availability of fully sequenced plant genomes, the progress in bioinformaticsresearch, development, or application of computational tools and approaches to explore biological data such as genome sequences or proteomics data to predict membrane transporters localized in plastids, and the development of highly sensitive proteomicssystematic, comprehensive analysis of the full set of proteins (the proteome) in an organism, a specific cell or tissue, or a cellular fraction such as internal membrane systems techniques open new avenues toward identifying additional, to date unknown, plastid envelope membrane transporters.