Transport into and out of the cell nucleus

A hallmark of eukaryotic cells is their separation into compartments. These compartments are surrounded by membranes that are impermeable to macromolecules. As most proteins are synthesised in the cytoplasm, specific transport systems have evolved to allow proteins to be imported from the cytoplasm into these compartments. I have been fascinated by this intracellular trafficking from the very beginning of my scientific life and had my first practical encounter with the problem in 1987 during a student project. This was in T.A.Rapoport's laboratory where, later, I also did my diploma and Ph.D. work on protein import into the rough endoplasmic reticulum (rER). During this time I identified and purified some of the key components of the machinery that translocates, for example, secretory proteins into the rER (Gorlich et al ., 1992a,b), and was finally able to reconstitute this ’translocon‘ from purified proteins and lipids (Gorlich and Rapoport, 1993). After this, I began working on nucleocytoplasmic transport, from 1993 in R.A.Laskey's laboratory (Cambridge, UK) and from 1996 with my own laboratory at the ZMBH in Heidelberg. This review will focus on nuclear transport; however, to begin, a brief comparison of import into the various compartments might be useful.Import, for example into mitochondria, chloroplasts or the rER, requires specific targeting sequences that are usually removed during import (for review see Rapoport et al ., 1996; Schatz and Dobberstein, 1996). Once a protein has been imported, for example, into the rER, there is normally no return to the cytoplasm. In contrast, in the case of nuclear transport, import and export are equally major processes (for review see Gorlich, 1997; Nigg, 1997; Mattaj and Englmeier, 1998). Many proteins need to be imported into the nucleus multiple times, as is the case for proteins which shuttle continuously between nucleus and cytoplasm. In …