Signaling pathways consist of a chain of biochemical events that form the intracellular equivalent of a fire bucket brigade. An initial change or chemical signal outside the cell is sensed at the cell surface by a receptor, which then transduces the signal to the cytosol. Over the last few years, investigations have focused increasingly on the spatiotemporal aspects of signaling, and it has become clear that the localization of key signaling components is highly regulated during signal

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... ion. Most signal transduction pathways cause specific changes in gene expression. Thus, an extracellular signal must be transduced across the plasma membrane and subsequently across the nuclear envelope to propagate the signal from the cytosol to the nucleus. Many signaling responses, therefore, rapidly effect the nuclear localization of transcription factors or alternatively of kinases that, once translocated, phosphorylate and activate transcription factors in the nucleus. Proteins travel into and out of the nucleus exclusively through the nuclear pore complex, an elaborate constellation of at least 30 distinct components embedded in the nuclear envelope (1). The mechanism by which proteins travel through the nuclear pore is the subject of much investigation (1); however in the past few years several aspects of nuclear transport have been clarified. Small molecules (less than 50,000 daltons) diffuse freely in and out of the nucleus through nuclear pores. To gain access to the nucleus, larger proteins require a nuclear localization sequence (NLS), 1 a series of basic residues that mediates binding to a protein called either a karyopherin or importin. The karyopherin-cargo complex is actively transported through the nuclear pores. Similarly, large proteins containing a nuclear export sequence (NES) bind to a different karyopherin protein, also called an exportin, and exit the nucleus. Other regulatory proteins, including the small GTPase RAN1, are required for trafficking in and out of the nucleus and ensure the directionality of transport (1). Signaling can induce the rapid redistribution of a protein from the cytosol to the nucleus by a number of different mechanisms. Two major factors that affect protein partitioning between the cytosol and nucleus are interactions with the nuclear transport machinery and interactions with anchor proteins that reside stably in the nucleus or cytosol. In the first case, signaling may directly regulate the association of a protein with nuclear import and/or export factors. Thus, a protein, the distribution of which is cytosolic and exhibits a low rate of nuclear import relative to its rate of nuclear export, will rapidly translocate to the nucleus in response to a regulated increase in its association with an importin, a regulated decrease in its association with an exportin, or a combina-tion of these two effects. YAP1 and nuclear factor of activated T cells (NFAT) exhibit this type of regulation. Alternatively a protein may contain an NLS but fail to enter the nucleus at a significant rate because of a high affinity interaction with a cytosolic anchor protein that does not enter the nucleus and may even occlude the NLS of its binding partner. Similarly, a protein may reside stably in the nucleus even though it contains an NES if it interacts tightly with a nuclear protein. In either case, the intracellular distribution of such a protein can be regulated by signal-mediated changes in its affinity for or the availability of its anchor(s). NF-B and MAPK illustrate this type of control. YAP1 Family of Transcription Factors Yap1p is a transcription factor found in the budding yeast, Saccharomyces cerevisiae, that allows these cells to respond to oxidative stress. Yap1p is related to c-Fos and c-Jun (AP1) in mammalian cells and the homologous transcription factors, PAP1 in fission yeast (Schizosaccharomyes pombe) and CAP1 in Candida albicans. In S. cerevisiae, Yap1p is found primarily in the cytosol, but after exposure of cells to a number of different oxidizing agents, such as diamide and diethylmaleate, the protein rapidly accumulates in the nucleus (2). Yap1p localization is regulated primarily at the level of nuclear export. In the absence of the exportin Xpo1p, the S. cerevisiae homologue of the mammalian Crm1 exportin, the protein is constitutively nuclear and transcribes its target genes in the absence of oxidizing agents (3, 4). The NES of Yap1p is localized to a highly conserved carboxyl-terminal domain, termed the cysteine-rich domain, that is both necessary and sufficient for oxidation-induced nuclear translocation (3, 4). Yap1p binds to Xpo1p in vivo and in vitro, and oxidation-induced relocalization of Yap1p seems to occur by direct inactivation of its NES. A group of conserved cysteine residues adjacent to the NES is required for regulated localization of Yap1p, and oxidation of these cysteines disrupts binding of Yap1p to Xpo1p in vitro (4). In S. pombe and C. albicans, the respective transcription factors PAP1 and CAP1 are similarly regulated, although PAP1 also requires the activity of the Spc1/Sty1 MAPK, an upstream component of the oxidative stress response, for its nuclear localization (5-7). Together the YAP1 family of transcription factors illustrates an elegant and direct method by which a change in environmental conditions leads to nuclear localization of a critical transcription factor through a regulated decrease in its rate of nuclear export.