Protein sorting and vesicle traffic

To function properly, a plant cell must direct many thousands of different polypeptides to specific metabolic compartments, cytoplasmic structures, and membrane systems. Membrane bound proteins occur in more than a dozen organelles and compartments, including the vacuolar membrane or tonoplast, the plasma membrane, membranes of the endoplasmic reticulum (ER), Golgi apparatus and peroxisomes, outer and inner envelopes of the chloroplast and mitochondria, and in thylakoid membranes. Soluble proteins are present in all subcellular compartments, including the cell wall and, with the only possible exception, the lumen of Golgi cisternae. Some proteins are unique to a particular structure, compartment, or membrane; other, very similar, proteins with comparable amino acid sequences, structures, and functions occur in more than one compartment. For example, invertases occur in the vacuole and cell wall, whereas water-channel proteins (aquaporins) are found in the tonoplast, plasma membrane and ER. Cells therefore require the necessary machinery to sort each protein and direct it to its proper destination.