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Abstract

Nucleo-cytoplasmic transport (net) is a particularly complex process (see the article of Paine, this volume, for a comprehensive analysis of protein traffic). Transport across the nuclear envelope is an important but by no means only aspect. Other aspects of net relate, for instance, to ana-and catabolic processes in both cytoplasm and nucleus. A further, rather crucial parameter is the association with and the possible transport on filamenteous or membraneous structures. Therefore, when trying to analyze the redistribution of a molecular species between cytoplasm and nucleus, the goal is, on the one hand, to exclude interference from biosynthesis and degradation, and, on the other hand, to discriminate between different categories of molecular transport, e.g. between random thermal motion in aqueous phases (diffusion), directed transport in aqueous phases (flow) I self-association, association with intracellular structures, and the random or directed, energy-dependent transport on intracellular interfaces. Last but not least, there is the microcosmos of membrane transport, i.e. the diffusional mediated or energy-dependent transport across intracellular membranes.