Freight management in the cell: current aspects of intracellular membrane trafficking

The correct transport of each protein to its respective location in the cell is a major task of eukaryotic cells. This problem is tackled by the employment of a large variety of cellular factors which, in a concerted action, recognize cargo proteins, translocate them across or insert them into specific membranes, sort them into transport vesicles, and confer the correct delivery to their final destinations. Like in an airport's cargo sorting system, the freight typically passes through several sequential sorting stations until it finally reaches the location that is specified by its individual address label. Although each membrane system employs its specific set of factors, the transport processes typically rely on common principles. Over the last years, many of these principles and the players involved were discovered. These findings allowed a rather detailed understanding of several transport processes like those into the endoplasmic reticulum (ER), the mitochondria, and the nucleus, or of the process of vesicular trafficking between the ER and the Golgi apparatus. With the advent of genome- and proteome-wide screens, a better understanding of the structures of the components involved and detailed studies of their dynamics, the future promises a broad comprehensive picture of the processes by which eukaryotic cells sort their proteins.