Nucleocytoplasmic shuttling signals: two for the price of one

It has been appreciated for some time that basic-amino-acid-type nuclear lo calization signals control nuclear uptake of proteins and that leucine-rich nuclear export signals mediate export back into the cytoplasm. The machine ry that recognizes and escorts these well-refined protein? transport signal s through the nuclear pore complex has been identified and characterized. D oes 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 nucleocyt oplasmic shuttling (NS) signals. All proteins currently known to contain th is type of signal also associate with mRNA. This article reviews what is cu rrently known about mediators of NS signal transport and discusses the link between NS signal-containing proteins and RNA export.