CDK1-Cyclin B1 Activates RNMT, Coordinating mRNA Cap Methylation with G1 Phase Transcription.

The creation of translation-competent mRNA is dependent on RNA polymerase II transcripts being modified by addition of the 7-methylguanosine (m7G) cap. The factors that mediate splicing, nuclear export, and translation initiation are recruited to the transcript via the cap. The cap structure is formed by several activities and completed by RNMT (RNA guanine-7 methyltransferase), which catalyzes N7 methylation of the cap guanosine. We report that CDK1-cyclin B1 phosphorylates the RNMT regulatory domain on T77 during G2/M phase of the cell cycle. RNMT T77 phosphorylation activates the enzyme both directly and indirectly by inhibiting interaction with KPNA2, an RNMT inhibitor. RNMT T77 phosphorylation results in elevated m7G cap methyltransferase activity at the beginning of G1 phase, coordinating mRNA capping with the burst of transcription that occurs following nuclear envelope reformation. RNMT T77 phosphorylation is required for the production of cohort of proteins, and inhibiting T77 phosphorylation reduces the cell proliferation rate.

p38gamma regulates interaction of nuclear PSF and RNA with the tumour-suppressor hDlg in response to osmotic shock.

Activation of p38γ modulates the integrity of the complex formed by the human discs large protein (hDlg) with cytoskeletal proteins, which is important for cell adaptation to changes in environmental osmolarity. Here we report that, in response to hyperosmotic stress, p38γ also regulates formation of complexes between hDlg and the nuclear protein polypyrimidine tract-binding protein-associated-splicing factor (PSF). Following osmotic shock, p38γ in the cell nucleus increases its association with nuclear hDlg, thereby causing dissociation of hDlg-PSF complexes. Moreover, hDlg and PSF bind different RNAs; in response to osmotic shock, p38γ causes hDlg-PSF and hDlg-RNA dissociation independently of its kinase activity. These findings identify a novel nuclear complex and suggest a previously unreported function of p38γ, which is independent of its catalytic activity and could affect mRNA processing and/or gene transcription to aid cell adaptation to osmolarity changes in the environment.

ERK5 pathway regulates the phosphorylation of tumour suppressor hDlg during mitosis.

Human disc-large (hDlg) is a scaffold protein critical for the maintenance of cell polarity and adhesion. hDlg is thought to be a tumour suppressor that regulates the cell cycle and proliferation. However, the mechanism and pathways involved in hDlg regulation during these processes is still unclear. Here we report that hDlg is phosphorylated during mitosis, and we establish the identity of at least three residues phosphorylated in hDlg; some are previously unreported. Phosphorylation affects hDlg localisation excluding it from the contact point between the two daughter cells. Our results reveal a previously unreported pathway for hDlg phosphorylation in mitosis and show that ERK5 pathway mediates hDlg cell cycle dependent phosphorylation. This is likely to have important implications in the correct timely mitotic entry and mitosis progression.

Alternative ERK5 regulation by phosphorylation during the cell cycle.

ERK5 is a member of the mitogen-activated protein kinase (MAPK) family that, after stimulation, is activated selectively by dual phosphorylation in the TEY motif by MAPK kinase 5 (MEK5). ERK5 plays an important role in regulating cell proliferation, survival, differentiation and stress response. Moreover, it is involved in G2/M progression and timely mitotic entry. ERK5 is phosphorylated during mitosis, but the molecular mechanism by which it is regulated during this phase is still unclear. Here we show that although ERK5 is phosphorylated in mitosis, this does not occur on the activation motif (TEY), but at its C-terminal half. We have identified five sites of ERK5 phosphorylation in mitosis, two of them unknown. Furthermore, we demonstrate that ERK5 phosphorylation in mitosis is not MEK5-dependent, but rather, cyclin-dependent kinase (CDK)-dependent. Using a mutagenesis approach, we analysed the importance of the phosphorylated residues in ERK5 function; our evidence show that phosphorylation in mitosis of the residues identified inhibits ERK5 activity and regulates ERK5 shuttling from cytoplasm to the nucleus. These results reveal a previously unreported form of ERK5 regulation by phosphorylation and establish a link between CDK and ERK5 pathways during mitosis, which could be crucial for the correct progression of the cell cycle.

Differential activation of p38MAPK isoforms by MKK6 and MKK3.

All four members of the mammalian p38 mitogen-activated protein kinase (MAPK) family (p38alpha, p38beta, p38gamma and p38delta) are activated by dual phosphorylation in the TGY motif in the activation loop. This phosphorylation is mediated by three kinases, MKK3, MKK6 and MKK4, at least in vitro. The role of these MKK in the activation of p38alpha has been demonstrated in studies using fibroblasts that lack MKK3 and/or MKK6. Nonetheless, the physiological upstream activators of the other p38MAPK isoforms have not yet been reported using MKK knockout cells. In this study, we examined p38beta, gamma and delta activation by MKK3 and MKK6, in cells lacking MKK3, MKK6 or both. We show that MKK3 and MKK6 are both essential for the activation of p38gamma and p38beta induced by environmental stress, whereas MKK6 is the major p38gamma activator in response to TNFalpha. In contrast, p38delta activation by ultraviolet radiation, hyperosmotic shock, anisomycin or by TNFalpha is mediated by MKK3. Moreover, in response to osmotic stress, MKK3 and MKK6 are crucial in regulating the phosphorylation of the p38gamma substrate hDlg and its activity as scaffold protein. These data indicate that activation of distinct p38MAPK isoforms is regulated by the selective and synchronized action of two kinases, MKK3 and MKK6, in response to cell stress.

Proteolysis of the tumour suppressor hDlg in response to osmotic stress is mediated by caspases and independent of phosphorylation.

Human disc-large (hDlg) is a scaffold protein critical for the maintenance of cell polarity and adhesion. hDlg is a component of the p38gamma MAP kinase pathway, which is important for the adaptation of mammalian cells to changes in environmental osmolarity. Here we report a strong decrease in the levels of hDlg protein in the human epithelial cell line HeLa when exposed to osmotic shock. This is independent of the phosphorylation state of hDlg, is prevented by preincubating the cell with the caspase inhibitor z-VAD and is part of the apoptotic process triggered by cellular stress. Although, both caspase 3 and caspase 6 are strongly activated by osmotic shock, the time course of caspase 6 activation parallels hDlg degradation, suggesting that this caspase may be responsible for the proteolysis. Mutating hDlg Asp747 to Ala abolishes caspase-induced cleavage, but does not affect the early stage of apoptosis or cell attachment. Our findings show that osmotic stress triggers hDlg degradation through a mechanism different from the one mediated by proteasomes, and we identify hDlg as a caspase substrate during the apoptotic process, although its proteolysis may not be implicated in the progression of early apoptosis.

Modulation of sarcoplasmic reticulum Ca(2+)-ATPase by chronic and acute exposure to peroxynitrite.

The Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum (SERCA), an integral membrane protein, becomes irreversibly inactivated in vitro by the addition of a single bolus of peroxynitrite with a K(0.5) of 200-300 microm, and this results in a large decrease of the ATP-dependent Ca2+ gradient across the sarcoplasmic reticulum (SR) membranes. The inactivation of SERCA is raised by treatment of SR vesicles with repetitive micromolar pulses of peroxynitrite. The inhibition of the SERCA is due to the oxidation of thiol groups and tyrosine nitration. Scavengers that react directly with peroxynitrite, such as cysteine, reduced glutathione, NADH, methionine, ascorbate or Trolox, a water-soluble analog of alpha-tocopherol, afforded significant protection. However, dimethyl sulfoxide and mannitol, two hydroxyl radical scavengers, and alpha-tocopherol did not protect SERCA from inactivation. Our results showed that the target of peroxynitrite is the cytosolic globular domain of the SERCA and that major skeletal muscle intracellular reductants (ascorbate, NADH and reduced glutathione) protected against inhibition of this ATPase by peroxynitrite.