The discovery of nuclear export signals (NESs) in a number of proteins revealed the occurrence of signal-dependent transport of proteins from the nucleus to the cytoplasm. Although the consensus motif of the NESs has been shown to be a leucine-rich, short amino-acid sequence, its receptor has not been identified. A cytotoxin leptomycin B (LMB) has recently been suggested to inhibit the NES-mediated transport of Rev protein. Here we show that LMB is a potent and specific inhibitor of the NES-dependent nuclear export of proteins. Moreover, we have found a protein of relative molecular mass 110K (p110) in Xenopus oocyte extracts that binds to the intact NES but not to the mutated, non-functional NES. The binding of p110 to NES is inhibited by LMB. We show that p110 is CRM1, which is an evolutionarily conserved protein originally found as an essential nuclear protein in fission yeast and known as a likely target of LMB. We also show that nuclear export of a fission yeast protein, Dsk1, which has a leucine-rich NES, is disrupted in wild-type yeast treated with LMB or in the crm1 mutant. These results indicate that CRM1 is an essential mediator of the NES-dependent nuclear export of proteins in eukaryotic cells.

The mitogen-activated protein kinase (MAPK) cascade consisting of MAPK and its direct activator, MAPK kinase (MAPKK), is essential for signaling of various extracellular stimuli to the nucleus. Upon stimulation, MAPK is translocated to the nucleus, whereas MAPKK stays in the cytoplasm. It has been shown recently that the cytoplasmic localization of MAPKK is determined by its nuclear export signal (NES) in the near N-terminal region (residues 33-44). However, the mechanism determining the subcellular distribution of MAPK has been poorly understood. Here, we show that introduction of v-Ras, active STE11 or constitutively active MAPKK can induce nuclear translocation of MAPK in mammalian cultured cells. Furthermore, we show evidence suggesting that MAPK is localized to the cytoplasm through its specific association with MAPKK and that nuclear accumulation of MAPK is accompanied by dissociation of a complex between MAPK and MAPKK following activation of the MAPK pathway. We have identified the MAPK-binding site of MAPKK as its N-terminal residues 1-32. Moreover, a peptide encompassing the MAPK-binding site and the NES sequence of MAPKK has been shown to be sufficient to retain MAPK to the cytoplasm. These findings reveal the molecular basis regulating subcellular distribution of MAPK, and identify a novel function of MAPKK as a cytoplasmic anchoring protein for MAPK.

Summary Circulating leptin and insulin convey information regarding energy stores to the central nervous system, particularly the hypothalamus. Hypothalamic pro-opiomelanocortin (POMC) neurons regulate energy balance and glucose homeostasis and express leptin and insulin receptors. However, the physiological significance of concomitant leptin and insulin action on POMC neurons remains to be established. Here we show that mice lacking both insulin and LepRs in POMC neurons (Pomc-Cre, Leprflox/flox IRflox/flox mice) display systemic insulin resistance, which is distinct from the single deletion of either receptor. In addition, Pomc-Cre, Leprflox/flox IRflox/flox female mice display elevated serum testosterone levels and ovarian abnormalities resulting in reduced fertility. We conclude that direct action of insulin and leptin on POMC neurons is required to maintain normal glucose homeostasis and reproductive function.

M-phase-promoting factor (MPF), a complex of cdc2 and a B-type cyclin, is a key regulator of the G 2 /M cell cycle transition. Cyclin B1 accumulates in the cytoplasm through S and G 2 phases and translocates to the nucleus during prophase. We show here that cytoplasmic localization of cyclin B1 during interphase is directed by its nuclear export signal (NES)-dependent transport mechanism. Treatment of HeLa cells with leptomycin B (LMB), a specific inhibitor of the NESdependent transport, resulted in nuclear accumulation of cyclin B1 in G 2 phase. Disruption of an NES which has been identified in cyclin B1 here abolished the nuclear export of this protein, and consequently the NES-disrupted cyclin B1 when expressed in cells accumulated in the nucleus. Moreover, we show that expression of the NES-disrupted cyclin B1 or LMB treatment of the cells is able to override the DNA damage-induced G 2 checkpoint when combined with caffeine treatment. These results suggest a role of nuclear exclusion of cyclin B1 in the DNA damage-induced G 2 checkpoint.

Mitogen-activated protein kinase (MAPK) is activated in cytoplasm in response to extracellular signals and then is translocated to nucleus. A directed activator for MAPK, MAPK kinase (MAPKK), stays in cytoplasm to transmit the signal from the plasma membrane to MAPK. Here we show that MAPKK contains a short amino acid sequence in the N-terminal region (residues 32-44), which acts as a nuclear export signal (NES) and thus is required for cytoplasmic localization of MAPKK. This NES sequence of MAPKK, like that of protein kinase inhibitor of cAMP-dependent protein kinase or Rev, is rich in leucine residues, which are crucial for the NES activity. Furthermore, the NES peptide of protein kinase inhibitor, as well as the NES peptide of MAPKK, inhibited the nuclear export of ovalbumin conjugated to the NES peptide of MAPKK. These results may suggest a common mechanism of nuclear export using a general leucine-rich NES.Mitogen-activated protein kinase (MAPK) 1 is activated in response to a wide variety of extracellular stimuli (1-4) and functions as one of several important mediators of signal transductions that control cell proliferation (5-10), cell differentiation (7,11,12), and early embryonic development (13-15). Activation of MAPK requires its dual phosphorylation on threonine and tyrosine residues catalyzed by MAPK kinase (MAPKK), a dual-specificity protein kinase (16,17). MAPKK exists in cytoplasm (18 -20) and is activated by serine phosphorylation (21-23) catalyzed by an upstream serine/threonine kinase, such as Raf-1 (24 -26), which may be activated near the plasma membrane (27-30). Thus, MAPKK is a key intermediate in the MAPK cascade, and cytoplasmic localization of MAPKK may be important for the proper signal transduction of the MAPK cascade. In fact, MAPK is first activated in cytoplasm through activation of MAPKK and then translocated to the nucleus (18,31,32). However, it remains unclear how cytoplasmic localization of MAPKK is achieved.The first identification of nuclear export signal (NES) (33-38) was recently done in studies characterizing two specific proteins (human immunodeficiency virus, type I-coded Rev protein and inhibitor (PKI) of cAMP-dependent protein kinase) that rapidly shuttle between the nucleus and the cytoplasm. The NES sequences in Rev and PKI are both rich in hydrophobic residues, in which three leucine residues are critical for nuclear export activity (33)(34)(35)(36)(37)(38).During the course of experiments originally designed to elucidate the mechanism of the nuclear translocation of MAPK, we found that MAPKK has in its N-terminal region a short sequence that regulates its subcellular distribution and that the sequence has an NES activity. This NES sequence of MAPKK, like the NES of PKI or Rev, is rich in leucine residues, which were found to be crucial for its NES activity. Furthermore, the NES of PKI and the NES of MAPKK competed with each other. These results may suggest a common mechanism of nuclear export using a general leucine-rich NES. MATERIALS AND METHODSDNA Construct...