TRANSPORT OF MACROMOLECULES BETWEEN THE NUCLEUS AND THE CYTOPLASM

Nuclear transport is an energy-dependent process mediated by saturable receptors, Import and export receptors are thought to recognize and b ind to nuclear localization signals or nuclear export signals, respect ively, in the transported molecules, The receptor-substrate interactio n can be direct or mediated by an additional adapter protein. The tran sport receptors dock their cargoes to the nuclear pore complexes (NPC) and facilitate their translocation through the NPC, After delivering their cargoes, the receptors are recycled to initiate additional round s of transport. Because a transport event for a cargo molecule is unid irectional, the transport receptors engage in asymmetric cycles of tra nslocation across the NPC, The GTPase Ran acts as a molecular switch f or receptor-cargo interaction and imparts directionality to the transp ort process. Recently, the combined use of different in vitro and in v ivo approaches has led to the characterization of novel import and exp ort signals and to the identification of the first nuclear import and export receptors.