Nucleocytoplasmic shuttling signals: two for the price of one

It has been appreciated for some time that basic-amino-acid-type nuclear localization signals control nuclear uptake of proteins and that leucine-rich nuclear export signals mediate export back into the cytoplasm. The machinery that recognizes and escorts these well-defined protein transport signals through the nuclear pore complex has been identified and characterized. Does this mean that the nuclear transport field knows all it needs to about transport signals? Not quite, as several recent publications have expanded the membership of a growing family of transport signals, known as nucleocytoplasmic shuttling (NS) signals. All proteins currently known to contain this type of signal also associate with mRNA. This article reviews what is currently known about mediators of NS signal transport and discusses the link between NS signal-containing proteins and RNA export.