No abstract

Checkpoint responses change as cells proceed through the cell cycle. Here we describe a novel checkpoint gene in fission yeast, mrc1 (mediator of replication checkpoint), that confers activation of the checkpoint kinase Cds1 to DNA synthesis (S) phase. Mrc1 associates with Cds1 and is required for regulation of Cds1 by the checkpoint kinase Rad3. Mrc1 is regulated by the cell cycle, with the appearance of Mrc1 mRNA and protein coinciding with S phase. We propose that coordinated expression of Mrc1 with replication control proteins helps to ensure activation of the appropriate checkpoint response during DNA replication.

Tropical vegetation is a major source of global land surface evapotranspiration, and can thus play a major role in global hydrological cycles and global atmospheric circulation. Accurate prediction of tropical evapotranspiration is critical to our understanding of these processes under changing climate. We examined the controls on evapotranspiration in tropical vegetation at 21 pan-tropical eddy covariance sites, conducted a comprehensive and systematic evaluation of 13 evapotranspiration models at these sites, and assessed the ability to scale up model estimates of evapotranspiration for the test region of Amazonia. Net radiation was the strongest determinant of evapotranspiration (mean evaporative fraction was 0.72) and explained 87% of the variance in monthly evapotranspiration across the sites. Vapor pressure deficit was the strongest residual predictor (14%), followed by normalized difference vegetation index (9%), precipitation (6%) and wind speed (4%). The radiation-based evapotranspiration models performed best overall for three reasons: (1) the vegetation was largely decoupled from atmospheric turbulent transfer (calculated from X decoupling factor), especially at the wetter sites; (2) the resistance-based models were hindered by difficulty in consistently characterizing canopy (and stomatal) resistance in the highly diverse vegetation; (3) the temperature-based models inadequately captured the variability in tropical evapotranspiration. We evaluated the potential to predict regional evapotranspiration for one test region: Amazonia. We estimated an Amazonia-wide evapotranspiration of 1370 mm yr À1 , but this value is dependent on assumptions about energy balance closure for the tropical eddy covariance sites; a lower value (1096 mm yr À1 ) is considered in discussion on the use of flux data to validate and interpolate models.

Unlike ubiquitin, the ubiquitin-like protein modifier SUMO-1 and its budding yeast homologue Smt3p have been shown to be more important for posttranslational protein modification than for protein degradation. Here we describe the identification of the SUMO-1 homologue of fission yeast, which we show to be required for a number of nuclear events including the control of telomere length and chromosome segregation. A disruption of the pmt3 ؉ gene, the Schizosaccharomyces pombe homologue of SMT3, was not lethal, but mutant cells carrying the disrupted gene grew more slowly. The pmt3⌬ cells showed various phenotypes such as aberrant mitosis, sensitivity to various reagents, and high-frequency loss of minichromosomes. Interestingly, we found that pmt3 ؉ is required for telomere length maintenance. Loss of Pmt3p function caused a striking increase in telomere length. When Pmt3p synthesis was restored, the telomeres became gradually shorter. This is the first demonstration of involvement of one of the Smt3p/SUMO-1 family proteins in telomere length maintenance. Fusion of Pmt3p to green fluorescent protein (GFP) showed that Pmt3p was predominantly localized as intense spots in the nucleus. One of the spots was shown to correspond to the spindle pole body (SPB). During prometaphase-and metaphase, the bright GFP signals at the SPB disappeared. These observations suggest that Pmt3p is required for kinetochore and/or SPB functions involved in chromosome segregation. The multiple functions of Pmt3p described here suggest that several nuclear proteins are regulated by Pmt3p conjugation.Ubiquitin is a small (76-residue), abundant protein conserved in all eukaryotic cells. It exists in several cellular compartments, such as the cytosol, nucleus, and cell surface. It is well known that ubiquitin regulates the function and stability of target proteins through its posttranslational conjugation to target proteins. Before conjugation to target proteins, ubiquitin must be processed by a C-terminal hydrolase. The first step of the ubiquitin conjugation pathway is the ATP-dependent formation of a thioester bond between the conserved C-terminal glycine of processed ubiquitin and the active-site cysteine residue of an E1 ubiquitin-activating enzyme. The second step is the transfer of activated ubiquitin to the active-site cysteine of an E2 ubiquitin-conjugating enzyme. In the final step, the E2 enzyme may cooperate with an E3 ubiquitin protein ligase to form an isopeptide bond between the C-terminal glycine of ubiquitin and the ε-amino groups of lysine residues of target proteins. Ubiquitin covalently conjugated to target proteins can be removed by a ubiquitin isopeptidase (89).Recently, a number of novel ubiquitin-like proteins were independently discovered in a number of species, suggesting that ubiquitin is part of a family of related proteins involved in the covalent modification of proteins. The first example of such a protein was the 15-kDa interferon-inducible, ubiquitin crossreacting protein UCRP (25). UCRP contains two ubiquitinr...

The PD-1:PDL pathway plays an important role in regulating alloimmune responses but its role in transplantation tolerance is unknown. We investigated the role of PD-1:PDL costimulatory pathway in peripheral and a well established model of central transplantation tolerance. Early as well as delayed blockade of PDL1 but not PDL2 abrogated tolerance induced by CTLA4Ig in a fully MHC-mismatched cardiac allograft model. Accelerated rejection was associated with a significant increase in the frequency of IFN-γ-producing alloreactive T cells and expansion of effector CD8+ T cells in the periphery, and a decline in the percentage of Foxp3+ graft infiltrating cells. Similarly, studies using PDL1/L2-deficient recipients confirmed the results with Ab blockade. Interestingly, while PDL1-deficient donor allografts were accepted by wild-type recipients treated with CTLA4Ig, the grafts developed severe chronic rejection and vasculopathy when compared with wild-type grafts. Finally, in a model of central tolerance induced by mixed allogeneic chimerism, engraftment was not abrogated by PDL1/L2 blockade. These novel data demonstrate the critical role of PDL1 for induction and maintenance of peripheral transplantation tolerance by its ability to alter the balance between pathogenic and regulatory T cells. Expression of PDL1 in donor tissue is critical for prevention of in situ graft pathology and chronic rejection.