Intracellular Targeting and SortingHow are macromolecules delivered to specific locations

T he ability of cells to deliver particular macromolecules to specific locations underlies the maintenance of functionally and architecturally distinct intracellular compartments-the cell's organelles, its surface domains, and specialized regions such as the axons and dendrites of neurons or the glycogen storage areas in the cytoplasm of hepatocytes. Cellular and molecular biologists currently are seeking common features of different molecules targeted to given sites. The features they seek can, when introduced into molecules or deleted from them (e.g., by genetic engineering), alter the destinations of the molecules. This approach, coupled to more traditional microscopic and biochemical analyses, have made it evident that targeting and sorting processes involve controls over both the delivery of individual molecules and the movement and behavior of multimolecular transport vehicles such as membrane-delimited vesicles. The principal known targeting mechanisms in eukaryotic cells include: