Unidirectional and Bidirectional Protein Traffic across Membranes

Intracellular protein topogenesis (Blobel 1980), i.e., the processes by which proteins are localized to their distinct cellular compartments and membranes, is a major activity of all cells, prokaryotic and eukaryotic. It was proposed some time ago that the information for protein topogenesis is encoded in discrete sequence elements (termed topogenic sequences) (Blobel and Sabatini 1971; Blobel and Dobberstein 1975a; Blobel 1980) of the protein to be localized. Because many proteins share localization to the same cellular compartment or membrane, they also share those topogenic sequences that direct them there. Many of these topogenic sequences have been deciphered. Each is decoded by a cognate effector system that results in localization of the protein to the specified cellular compartment or membrane.