Nucleocytoplasmic transport: Ran, beta and beyond

Proteins, RNAs and even large macromolecular complexes are transported into and out of nuclei with remarkable rapidity and specificity. Nucleocytoplas mic transport must therefore be efficient and selective, Characterization o f the roles of the importin beta family of transport receptors and of the R an GTPase has showed how these characteristics can be achieved, but there a re many examples of nucleocytoplasmic transport that do not fit this model. Here, we discuss current understanding of various transport mechanisms and evaluate cases in which the molecules and mechanisms underlying nucleocyto plasmic transport are used to carry out important cellular functions in the absence of a nucleus.