Peptidyl-prolyl isomerases: a new twist to transcription

Peptidyl-prolyl isomerases (PPIs) catalyse the cis–trans isomerisation of peptide bonds N-terminal to proline residues in polypeptide chains. They have roles in the folding of newly synthesised proteins and in the function of the immune system. In addition, members of the parvulin-like family of PPIs have been implicated in cell cycle control. Their activity is directed by the prior phosphorylation of target proteins in both yeast and mammalian cells. More recent data have illustrated that they may also influence other nuclear events. This review examines PPI activity in the context of eukaryotic transcriptional regulation. The findings are consistent with a two-step model of conformational control, in which the outcome depends on the transcription factor involved

Localization, Dynamics, and Function of Survivin Revealed by Expression of Functional SurvivinDsRed Fusion Proteins in the Living Cell

Survivin, a member of the inhibitor of apoptosis protein family, has attracted growing attention due to its expression in various tumors and its potential application in tumor therapy. However, its subcellular localization and function have remained controversial: Recent studies revealed that survivin is localized at the mitotic spindle, binds caspases, and could thus protect cells from apoptosis. The cell cycle-dependent expression of survivin and its antiapoptotic function led to the hypothesis that survivin connects the cell cycle with apoptosis, thus providing a death switch for the termination of defective mitosis. In other studies, survivin was detected at kinetochores, cleavage furrow, and midbody, localizations being characteristic for chromosomal passenger proteins. These proteins are involved in cytokinesis as inferred from the observation that RNA interference and expression of mutant proteins led to cytokinesis defects without an increase in apoptosis. To remedy these discrepancies, we analyzed the localizations of a survivinDsRed fusion protein in HeLa cells by using confocal laser scanning microscopy and time-lapse video imaging. SurvivinDsRed was excluded from the interphase nucleus and was detected in centrosomes and at kinetochores. It dissociated from chromosomes at the anaphase/telophase transition and accumulated at the ends of polar microtubuli where it was immediately condensed to the midbody. Overexpression of both survivinDsRed and of a phosphorylation-defective mutant conferred resistance against apoptosis-inducing reagents, but only the overexpressed mutant protein caused an aberrant cytokinesis. These data characterize in detail the dynamics of survivin in vertebrate cells and confirm that survivin represents a chromosomal passenger protein

Aurora B Kinase Exists in a Complex with Survivin and INCENP and Its Kinase Activity Is Stimulated by Survivin Binding and Phosphorylation

Aurora B regulates chromosome segregation and cytokinesis and is the first protein to be implicated as a regulator of bipolar attachment of spindle microtubules to kinetochores. Evidence from several systems suggests that Aurora B is physically associated with inner centromere protein (INCENP) in mitosis and has genetic interactions with Survivin. It is unclear whether the Aurora B and INCENP interaction is cell cycle regulated and if Survivin physically interacts in this complex. In this study, we cloned the Xenopus Survivin gene, examined its association with Aurora B and INCENP, and determined the effect of its binding on Aurora B kinase activity. We demonstrate that in the Xenopus early embryo, all of the detectable Survivin is in a complex with both Aurora B and INCENP throughout the cell cycle. Survivin and Aurora B bind different domains on INCENP. Aurora B activity is stimulated >10-fold in mitotic extracts; this activation is phosphatase sensitive, and the binding of Survivin is required for full Aurora B activity. We also find the hydrodynamic properties of the Aurora B/Survivin/INCENP complex are cell cycle regulated. Our data indicate that Aurora B kinase activity is regulated by both Survivin binding and cell cycle-dependent phosphorylation

The Schizosaccharomyces pombe Aurora–related Kinase Ark1 Interacts with the Inner Centromere Protein Pic1 and Mediates Chromosome Segregation and Cytokinesis

The chromosomal passenger proteins aurora-B, survivin, and inner centromere protein (INCENP) have been implicated in coordinating chromosome segregation with cell division. This work describes the interplay between aurora, survivin, and INCENP orthologs in the fission yeast Schizosaccharomyces pombe and defines their roles in regulating chromosome segregation and cytokinesis. We describe the cloning and characterization of the aurora-related kinase gene ark1(+), demonstrating that it is an essential gene required for sister chromatid segregation. Cells lacking Ark1p exhibit the cut phenotype, DNA fragmentation, and other defects in chromosome segregation. Overexpression of a kinase-defective version of Ark1, Ark1-K147R, inhibits cytokinesis, with cells exhibiting an elongated, multiseptate phenotype. Ark1p interacts physically and/or genetically with the survivin and INCENP orthologs Bir1p and Pic1p. We identified Pic1p in a two-hybrid screen for Ark1-K147R interacting partners and went on to map domains in both proteins that mediate their binding. Pic1p residues 925–972 are necessary and sufficient for Ark1p binding, which occurs through the kinase domain. As with Ark1-K147R, overexpression of Ark1p-binding fragments of Pic1p leads to multiseptate phenotypes. We also provide evidence that the dominant-negative effect of Ark1-K147R requires Pic1p binding, indicating that the formation of Ark1p-Pic1p complexes is required for the execution of cytokinesis

Tetrapyrrole Metabolism in Arabidopsis thaliana

Higher plants produce four classes of tetrapyrroles, namely, chlorophyll (Chl), heme, siroheme, and phytochromobilin. In plants, tetrapyrroles play essential roles in a wide range of biological activities including photosynthesis, respiration and the assimilation of nitrogen/sulfur. All four classes of tetrapyrroles are derived from a common biosynthetic pathway that resides in the plastid. In this article, we present an overview of tetrapyrrole metabolism in Arabidopsis and other higher plants, and we describe all identified enzymatic steps involved in this metabolism. We also summarize recent findings on Chl biosynthesis and Chl breakdown. Recent advances in this field, in particular those on the genetic and biochemical analyses of novel enzymes, prompted us to redraw the tetrapyrrole metabolic pathways. In addition, we also summarize our current understanding on the regulatory mechanisms governing tetrapyrrole metabolism. The interactions of tetrapyrrole biosynthesis and other cellular processes including the plastid-to-nucleus signal transduction are discussed