George Pyrowolakis

RAD6-dependent DNA repair is linked to modification of PCNA by ubiquitin and SUMO

The RAD6 pathway is central to post-replicative DNA repair in eukaryotic cells; however, the machinery and its regulation remain poorly understood. Two principal elements of this pathway are the ubiquitin-conjugating enzymes RAD6 and the MMS2–UBC13 heterodimer, which are recruited to chromatin by the RING-finger proteins RAD18 and RAD5, respectively. Here we show that UBC9, a small ubiquitin-related modifier (SUMO)-conjugating enzyme, is also affiliated with this pathway and that proliferating cell nuclear antigen (PCNA)—a DNA-polymerase sliding clamp involved in DNA synthesis and repair—is a substrate. PCNA is mono-ubiquitinated through RAD6 and RAD18, modified by lysine-63-linked multi-ubiquitination—which additionally requires MMS2, UBC13 and RAD5—and is conjugated to SUMO by UBC9. All three modifications affect the same lysine residue of PCNA, suggesting that they label PCNA for alternative functions. We demonstrate that these modifications differentially affect resistance to DNA damage, and that damage-induced PCNA ubiquitination is elementary for DNA repair and occurs at the same conserved residue in yeast and humans.

SUMO, UBIQUITIN'S MYSTERIOUS COUSIN

Covalent modification of cellular proteins by the ubiquitin-like modifier SUMO regulates various cellular processes, such as nuclear transport, signal transduction, stress response and cell-cycle progression. But, in contrast to ubiquitylation, sumoylation does not tag proteins for degradation, but seems to enhance their stability or modulate their subcellular compartmentalization.

Ubiquitin and its kin: how close are the family ties?

Modification of proteins by the covalent attachment of ubiquitin is known to target them for degradation by proteasomes. Several proteins have been discovered recently that are related to ubiquitin or function similarly. Some of these proteins act as modifiers; others bear ubiquitin-like domains embedded in their polypeptide chain but do not form conjugates with cellular proteins. Ubiquitin-like proteins mediate an impressive range of cellular functions, including cell-cycle progression, DNA repair and apoptosis. Recent discoveries endorse the view that, in many cases, the function of the relatives of ubiquitin is linked to the ubiquitin pathway.

Ubiquitin and proteasomes: Sumo, ubiquitin's mysterious cousin

Covalent modification of cellular proteins by the ubiquitin-like modifier SUMO regulates various cellular processes, such as nuclear transport, signal transduction, stress response and cell-cycle progression. But, in contrast to ubiquitylation, sumoylation does not tag proteins for degradation, but seems to enhance their stability or modulate their subcellular compartmentalization.

Conversion of an Extracellular Dpp/BMP Morphogen Gradient into an Inverse Transcriptional Gradient

Morphogen gradients control body pattern by differentially regulating cellular behavior. Here, we analyze the molecular events underlying the primary response to the Dpp/BMP morphogen in Drosophila. Throughout development, Dpp transduction causes the graded transcriptional downregulation of the brinker (brk) gene. We first provide significance for the brk expression gradient by showing that different Brk levels repress distinct combinations of wing genes expressed at different distances from Dpp-secreting cells. We then dissect the brk regulatory region and identify two separable elements with opposite properties, a constitutive enhancer and a Dpp morphogen-regulated silencer. Furthermore, we present genetic and biochemical evidence that the brk silencer serves as a direct target for a protein complex consisting of the Smad homologs Mad/Medea and the zinc finger protein Schnurri. Together, our results provide the molecular framework for a mechanism by which the extracellular Dpp/BMP morphogen establishes a finely tuned, graded read-out of transcriptional repression.

Control of Dpp morphogen signalling by a secreted feedback regulator.

In many instances during development, morphogens specify cell fates by forming concentration gradients. In the Drosophila melanogaster wing imaginal disc, Decapentaplegic (Dpp), a bone morphogenetic protein (BMP), functions as a long-range morphogen to control patterning and growth. Dpp is secreted from a stripe of cells at the anterior-posterior compartment boundary and spreads into both compartments to generate a characteristic BMP activity gradient. Ever since the identification of the morphogen activity of Dpp in the developing wing, the system has served as a paradigm to understand how long-range gradients are established and how cells respond to such gradients. Here we reveal the tight and direct connection of these two processes with the identification and characterization of pentagone (pent), a transcriptional target of BMP signalling encoding a secreted regulator of the pathway. Absence of pent in the wing disc causes a severe contraction of the BMP activity gradient resulting in patterning and growth defects. We show that Pent interacts with the glypican Dally to control Dpp distribution and provide evidence that proper establishment of the BMP morphogen gradient requires the inbuilt feedback loop embodied by Pent.

A conserved activation element in BMP signaling during Drosophila development

The transforming growth factor β (TGF-β) family member Decapentaplegic (Dpp) is a key regulator of patterning and growth in Drosophila development. Previous studies have identified a short DNA motif called the silencer element (SE), which recruits a trimeric Smad complex and the repressor Schnurri to downregulate target enhancers upon Dpp signaling. We have now isolated the minimal enhancer of the dad gene and discovered a short motif we termed the activating element (AE). The AE is similar to the SE and recruits the Smad proteins via a conserved mechanism. However, the AE and SE differ at important nucleotide positions. As a consequence, the AE does not recruit Schnurri but rather integrates repressive input by the default repressor Brinker and activating input by the Smad signal transducers Mothers against Dpp (Mad) and Medea via competitive DNA binding. The AE allows the identification of hitherto unknown direct Dpp targets and is functionally conserved in vertebrates.

Dpp Signaling Activity Requires Pentagone to Scale with Tissue Size in the Growing Drosophila Wing Imaginal Disc

The activity of the Dpp morphogen adapts to tissue size in the growing Drosophila wing imaginal disc, and Pentagone, an important secreted feedback regulator of the Dpp pathway, is required for this adaptation.

Expansion-Repression Mechanism for Scaling the Dpp Activation Gradient in Drosophila Wing Imaginal Discs

Maintaining a proportionate body plan requires the adjustment or scaling of organ pattern with organ size. Scaling is a general property of developmental systems, yet little is known about its underlying molecular mechanisms. Using theoretical modeling, we examine how the Dpp activation gradient in the Drosophila wing imaginal disc scales with disc size. We predict that scaling is achieved through an expansion-repression mechanism [1] whose mediator is the widely diffusible protein Pentagone (Pent). Central to this mechanism is the repression of pent expression by Dpp signaling, which provides an effective size measurement, and the Pent-dependent expansion of the Dpp gradient, which adjusts the gradient with tissue size. We validate this mechanism experimentally by demonstrating that scaling requires Pent and further, that scaling is abolished when pent is ubiquitously expressed. The expansion-repression circuit can be readily implemented by a variety of molecular interactions, suggesting its general utilization for scaling morphogen gradients during development.

BRUCE, a Giant E2/E3 Ubiquitin Ligase and Inhibitor of Apoptosis Protein of the trans-Golgi Network, Is Required for Normal Placenta Development and Mouse Survival

ABSTRACT BRUCE is a highly conserved 528-kDa peripheral membrane protein of the trans-Golgi network. Owing to the presence of an N-terminal single baculovirus inhibitor repeat, BRUCE functions as an inhibitor of apoptosis protein and blocks apoptosis when overexpressed. In addition, due to the presence of a C-terminal ubiquitin-conjugating domain, BRUCE can covalently attach ubiquitin to substrates. Here we report the generation and characterization of BRUCE-deficient mice. Complete inactivation of the BRUCE gene resulted in perinatal lethality and growth retardation discernible after embryonic day 14. The growth defect is linked to impaired placental development and may be caused by insufficient oxygen and nutrient transfer across the placenta. Chorioallantoic placentation initiated normally, but the mutant placenta showed an impaired maturation of the labyrinth layer and a significant reduction of the spongiotrophoblast. No evidence for an elevated apoptosis rate was detectable in embryonic and extraembryonic tissues and in knockout fibroblasts. Thus, although BRUCE is broadly expressed in embryonic, extraembryonic, and adult mouse tissues, this bifunctional protein might play a unique role in normal trophoblast differentiation and embryonic survival.