Signalling takes control of nucleo-cytoplasmic trafficking. Workshop on signal-regulated nuclear transport.

Regulated transit of macromolecules across the nuclear envelope (NE) is a hallmark of eukaryotic cells, and in the last 10 years basic transport mechanisms have been elucidated. Transport factors of the importin/karyopherin-β (Impβ) superfamily decode nuclear localization signals (NLS) or nuclear export signals (NES) in the amino acid sequence of proteins and chaperone them through the nuclear pore complex (NPC). Vectorial transport of proteins and some ribonucleoproteins (RNPs) is achieved due to two principal features of the transport machinery: the asymmetric localization of NPC proteins on the nuclear and the cytoplasmic faces of the pore; and the nucleocytoplasmic gradient of the small GTPase Ran, which controls the stability of Impβ interactions with particular cargo molecules. This meeting emphasized the important links between nucleocytoplasmic transport and signal transduction pathways involving small GTPases, kinases and phosphatases. Among the highlights were reports on the regulation of the subcellular localization of signal transducers and transcription factors and the suggestion that the nucleus itself is endowed with key components that are regulated during signal transduction, in particular Ca2+. Also exciting was the discussion of how large nucleic acids traffic to and across the NE. This holds the key to understanding the complexity of mRNA biogenesis and function and of viral trafficking. It emerged that nuclear export of bulk mRNAs follows somewhat different rules than the wellcharacterized Imp-mediated export of proteins, tRNAs or small nuclear (sn) RNAs and that certain viruses use different import mechanisms than do generic NLS-containing proteins. These findings suggest that the regulatory elements controlling housekeeping functions are distinct from those controlling atypical processes.