Each organelle of a eukaryotic cell contains a distinct set of proteins required for its function. These organelle-specific proteins are identified by localization signals and directed to their respective organelles by intracellular transport systems. Transporting proteins into the nucleus is a particularly interesting system, not only because a large volume of proteins is transported, but also because the activities of certain key regulatory proteins are modulated by controlling their entry into the nucleus. Therefore, regulating this process can produce dramatic effects on gene expression.

To take a nuclear protein from the cytoplasm through a nuclear pore and into the nucleus involves, as an early step, recognizing a nuclear localization signal. This signal is a sequence of usually basic amino acids possibly occurring anywhere within the primary sequence of a nuclear protein. Little is known about the cellular machinery that recognizes these nuclear localization signals and transports the proteins to the nucleus or how transport is controlled. To investigate these processes, we use biochemical approaches to identify proteins that are involved in nuclear localization and apply molecular and cellular biological tools along with physical methods to examine the mechanisms by which they function.