Selective movement of proteins between the nucleus and the cytoplasm is a regulatory mechanism exploited extensively by the eukaryotic cell. We have identified the evolutionarily conserved Sac3 protein, which was implicated previously in the regulation of mitosis [Bauer, A. & Kö lling, R. (1996) J. Cell Sci. 109, 1575-1583] as a novel mediator of nuclear protein export. We show that Sac3p is localized to the nuclear pore, where it interacts with nucleoporins. Loss of SAC3 function results in a block in nuclear export of a nuclear export signal-containing reporter protein. Our results also demonstrate that SAC3 interacts genetically with the nuclear protein export factors Crm1p͞Xpo1p and Yrb2p. Taken together, these data indicate a link between nuclear protein export and transition through the cell cycle. The nuclear envelope provides a physical mechanism for regulation of numerous events in the eukaryotic cell. The segregation of essential cellular processes such as transcription and translation conferred by the separation of the nucleus and the cytoplasm allows cells to use compartmentalization as a method for controlling critical cellular processes. Efficient use of this regulatory mechanism requires rapid but highly regulated pathways for exchange of information between the nucleus and the cytoplasm. The route for this exchange in all cells is via nuclear pore complexes that form large proteinaceous channels in the double membrane that surrounds the nucleus (1). In addition to the nuclear pore proteins, which are called nucleoporins, a growing number of soluble factors are required to escort substrates through the nuclear pore complex (2). These factors can be divided into two general classes: those that target substrates to the nuclear pore, including the soluble receptors of the importin family (3), and those that are required for the actual translocation through the pore, primarily the small, GTPbinding protein Ran and its binding partner NTF2 (4, 5).For many years, studies have focused on how entry of proteins into the nucleus can regulate cellular responses to external stimuli. Several recent developments have led researchers to consider the potential impact of selective export of proteins from the nucleus. Approximately 5 years ago, a nuclear export signal (NES) that targets substrate proteins for export from the nucleus was defined (6). This discovery was followed rapidly by the identification of an NES receptor called Xpo1p͞Crm1p in Saccharomyces cerevisiae (7,8) and CRM1 in other eukaryotes (9). The nuclear export receptor is a member of a large family of proteins related to the importin ␤-subunit of the classical nuclear localization signal (NLS) receptor. Despite these recent advances, the mechanism of nuclear protein export is not understood in detail. Thus far, only one additional soluble factor has been implicated in the process, the Ran-binding protein Yrb2p (10,11).Interest in the detailed mechanism of nuclear protein export has been sparked further by several recent studies that demonstrate t...