The mechanism of senescence-associated cytoplasmic induction of p-Erk1/2 (SA-p-Erk1/2) proteins in human diploid fibroblasts was investigated. p-Erk1/2 proteins were efficiently dephosphorylated in vitro by protein phosphatases 1 and 2A (PP1/2A) and MAPK phosphatase 3 (MKP3). Specific activity of PP1/2A and MKP3 activity significantly decreased during cellular senescence, whereas their protein expression levels did not. To investigate possible mechanism of phosphatase inactivation, we measured reactive oxygen species (ROS) generation by fluorescence-activated cell sorting analysis and found it was much higher in mid-old cells than the young cells. Decreased growth rate, limited cell division, flat and large cell shapes, and tight binding of the cells to culture dished (1, 2) are well known characteristics of cells entering into replicative senescence (3). Primary mouse embryo fibroblasts exposed to oncogenic Ras overexpression undergo premature senescence in response to constitutive MAPK 1 kinase/MAPK mitogenic signaling (4, 5), whereas established variants lacking p53 or p19 ARF are efficiently transformed (6). Ha-Ras mutants that interact preferentially with specific Ras effector proteins are known as V 12 S 35 , V 12 C 40 , and V 12 G 37 (7, 8); V 12 S 35 binds preferentially to Raf-1 and activates MAPK without any effect on membrane ruffling (7), and the V 12 C 40 associates with phosphatidylinositol 3-kinase (Akt kinase) and promotes membrane ruffling and cell survival independent of Raf-1 (8, 9), but the V 12 G 37 binds to Ral-GDS without binding to Raf-1 or phosphatidylinositol 3-kinase (7). A distinctive feature of the cellular senescence induced by overexpression of the Ha-ras mutants as well as the replicative senescence in normal human diploid fibroblasts (HDF) is the markedly increased phosphorylation of extracellular signal-regulated protein kinase (pErk1/2) without nuclear translocation (10). However, its biochemical mechanism has not yet been clarified.MAPK activity is tightly regulated by phosphorylation and dephosphorylation. The activation of the MAPK activity requires the dual phosphorylation of the Ser/Thr and Tyr residues in the TXY kinase activation motif (11-13), and deactivation occurs through the action of either Ser/Thr protein phosphatase (14), protein-tyrosine phosphatase (PTP) (14, 15), or dual specificity phosphatases (16,17). The dual phosphatases are capable of catalyzing the removal of the phosphoryl group from Tyr(P) as well as Ser(P)/Thr(P) residues. The dual specificity phosphatases that specifically dephosphorylate and inactivate the p-Erk1/2 are called MAPK phosphatases (MKPs), and at least 9 mammalian MKPs have been identified so far (18,19). It has been reported that MKP3 (20), also termed Pyst1 (21) or VH6 (22), is predominantly localized in the cytoplasm, is highly specific for Erk1/2 inactivation, and is not inducible by either growth factor or stress (20 -23). An elegant series of studies have shown that the N-terminal domain of MKP3 can physically associate with Erk1/...

It has been shown previously that the oxidative folding of bovine pancreatic ribonuclease A proceeds through parallel pathways with two major native-like three-disulfide (3S) intermediates. We show here that, under some conditions, the native disulfide bonds can also be regenerated through disproportionation reactions; in other words, the protein can serve as its own redox reagent. The results also show that disulfide species of the unstructured 3S ensemble have a strong propensity to participate in intermolecular interactions. These interactions are favored at high protein concentration, temperature and pH, and lead to formation of the native structure during disulfide reshuffling in the rate-determining step.z 2000 Federation of European Biochemical Societies.

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