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The exportin CRM1 binds nuclear export signals (NESs), and mediates active transport of NES-bearing proteins from the nucleus to the cytoplasm. Structural and biochemical analyses have uncovered the molecular mechanisms underlying CRM1/NES interaction. CRM1 binds NESs through a hydrophobic cleft, whose open or closed conformation facilitates NES binding and release. Several cofactors allosterically modulate the conformation of the NES-binding cleft through intramolecular interactions involving an acidic loop and a C-terminal helix in CRM1. This current model of CRM1-mediated nuclear export has not yet been evaluated in a cellular setting. Here, we describe SRV100, a cellular reporter to interrogate CRM1 nuclear export activity. Using this novel tool, we provide evidence further validating the model of NES binding and release by CRM1. Furthermore, using both SRV100-based cellular assays and in vitro biochemical analyses, we investigate the functional consequences of a recurrent cancer-related mutation, which targets a residue near CRM1 NES-binding cleft. Our data indicate that this mutation does not necessarily abrogate the nuclear export activity of CRM1, but may increase its affinity for NES sequences bearing a more negatively charged C-terminal end. [ABSTRACT FROM AUTHOR]

The mitogen-activated protein kinase-activated protein kinase-5 (MK5) resides predominantly in the nucleus of resting cells, but p38, extracellular signal-regulated kinases-3 and -4 (ERK3 and ERK4), and protein kinase A (PKA) induce nucleocytoplasmic redistribution of MK5. The mechanism by which PKA causes nuclear export remains unsolved. In the study reported here we demonstrated that Ser-115 is an in vitro PKA phosphoacceptor site, and that PKA, but not p38, ERK3 or ERK4, is unable to redistribute MK5 S115A to the cytoplasm. However, the phosphomimicking MK5 S115D mutant resides in the cytoplasm in untreated cells. While p38, ERK3 and ERK4 fail to trigger nuclear export of the kinase dead T182A and K51E MK5 mutants, S115D/T182A and K51E/S115D mutants were able to enter the cytoplasm of resting cells. Finally, we demonstrated that mutations in Ser-115 affect the biological properties of MK5. Taken together, our results suggest that Ser-115 plays an essential role in PKA-regulated nuclear export of MK5, and that it also may regulate the biological functions of MK5. [ABSTRACT FROM AUTHOR]

Poly(ADP-ribose) glycohydrolase (PARG) is a catabolic enzyme that cleaves ADP-ribose polymers formed by members of the PARP family of enzymes. Despite its discovery and subsequent partial purification in the 1970s [1-3] and the cloning of its single gene in the late 1990s [4], little is known about the role of PARG in cell function. Because of its low abundance within cells and its extreme sensitivity to proteases, PARG has been difficult to study. The existence of several PARG isoforms with different subcellular localizations is still debated today after more than 30 years of intensive research. In this article, we want to summarize and discuss the current knowledge related to PARG, its different forms and subcellular distribution. We also examine the possible biological roles of PARG in modulating chromatin structure, transcription, DNA repair and apoptosis. [ABSTRACT FROM AUTHOR]

The nuclear factors of activated T cells (NF-ATs) constitute a family of transcription factors that transduce calcium signals in the immune, cardiac, muscular and nervous systems. Like their distant relatives of the Rel family, including NF-κB, NF-ATs are cytoplasmic in resting cells and activated by means of induced nuclear import. Unlike NF-κB, however, NF-ATs show highly dynamic nuclear shuttling properties that have important implications for graded signaling by these molecules. This review focuses on recent advances in deciphering mechanisms by which calcium signaling regulates the nucleo-cytoplasmic shuttling and therefore transactivation functions of the NF-ATs. These discoveries highlight the interplay between nuclear import and export signals on NF-ATs, and the roles of the calcium-activated phosphatase calcineurin and NF-AT kinases in controlling the activity of these signals. They also reveal that NF-ATs, as well as other transcription factors controlled at the level of nuclear import, face the very real prospect of futile cycling across the nuclear envelope as a consequence of conflicting nuclear import and export signals. We discuss the molecular mechanisms by which calcineurin suppresses futile cycling, as well as the major challenges to our understanding of NF-AT signaling in diverse biological systems. [ABSTRACT FROM AUTHOR]