Ubiquitin

Ubiquitin is a protein (ie ubiquitous ) found in all eukaryotic cells. The protein changes the properties of other proteins to which it is reversible ( invertible ) bound. Depending on the type and number of ubiquitin bonds whose half-life, function or distribution within the cell ( localization ) are thereby regulated. The ubiquitin system is involved, among others, the protein quality control, signal transduction and cell cycle control. It is thus the change already -translated proteins ( posttranslational modification) and may further be involved in the development of various diseases. Together with proteins such as SUMO1, Nedd8, and other part of the ubiquitin ubiquitin-protein family. Recent findings suggest that certain prokaryotes have analogous to ubiquitin protein.

For the study of the fundamentals of the ubiquitin system in the early 1980s, Aaron Ciechanover, Avram Hershko and Irwin Rose were awarded the 2004 Nobel Prize in Chemistry.

Structure

Ubiquitin consists of 76 amino acids and has a molecular mass of 8.5 kDa. Its construction changed little during evolution, it is thus highly conserved. Thus, the protein differ in humans and in the unicellular yeast Saccharomyces cerevisiae in only 3 of the 76 amino acids.

Ubiquitin is a globular form, only the last four C -terminal amino acids protrude. As functional amino acids are the C-terminal glycine at the 76th place ( G76 ) and the lysines in the 48th and 63rd of the amino acid sequence ( K48 and K63 ). On the glycine ubiquitin is covalently bound to the protein to be labeled. About the Lysine further ubiquitin molecules can be added to an already bound ubiquitin and ubiquitin as a are formed. Since ubiquitin contains seven lysines, at least seven different types of connection of a ubiquitin are possible.

The human amino acid sequence in single letter code for ubiquitin ( K48, K63 and G76 are highlighted):

Mechanism of ubiquitination

The process of labeling of target proteins is called ubiquitination or ubiquitylation. Accordingly, we speak in the modification of proteins by SUMO1, Urm1 and Nedd8 of sumoylation, Urmylierung and Neddylierung. The mechanism is the same for all three proteins and is of three enzymes catalyzes the reaction sequence according to the E1 (also modification activating enzyme ), E2 (also modification conjugating enzyme ) and E3 (including E3 ligase ) are referred to. Ubiquitin in the first step is bound ( through a thioester bond ) between a cysteine ​​residue of the enzyme E1, and the C-terminal carboxyl group of ubiquitin, thereby "activated". This activation is energy dependent; the energy is supplied by the cleavage of ATP to AMP and pyrophosphate. For each modified protein, there is exactly one E1 enzyme, which is specific for its activation ( only in plants there for two ubiquitin E1 enzymes). After ubiquitin is bound to E1, the ubiquitin to the E2 enzyme is transferred. During exists for SUMO1 and Nedd8 per a specific E2 enzyme, various E2 enzymes are known for ubiquitin alone in the yeast over eleven. In higher organisms, their number is larger. In the last step, the ubiquitin is transferred by the E3 ligase, to the target protein. Here, an isopeptide bond between a lysine of the target protein and the C- terminal glycine of ubiquitin is formed. It differs from a conventional peptide bond, characterized in that not the α - amino group, but the ε - amino group of lysine is used as a binding partner. Deviating was also closed to other types of links, as well as lysine - free proteins were found ubiquitinated. The variety of modified ubiquitin target proteins is reflected in the number of different E3 enzymes. Taking into account all the enzymes structurally to the three sub-families of E3 enzymes ( HECT, RING and U -box) belong, is in higher organisms by a number between several hundred and thousand to go.

Types of ubiquitination

Depending on the number of connected ubiquitin molecules, a distinction between a mono-, oligo -, multi- and poly - ubiquitination. If at least five ubiquitin molecules are connected as a chain with a target protein, it is called a poly - ubiquitination. With this, the molecules are linked by the lysine 48, the target protein is mainly supplied to the degradation by the proteasome. Connection with lysine 63 may lead to lysosomal degradation of the protein. Furthermore, it was observed that this modification has an effect on cellular DNA damage tolerance, inflammatory immune responses endocytotic processes and ribosomal protein synthesis. Mono- and multi- ubiquitylations however, less affect the stability of individual proteins as their intracellular distribution and can allow interaction with other proteins. Oligo ubiquitination has, for example, influence on the activity of a transcription factor to initiate without its decomposition.

Examples of ubiquitylations

Degradation faulty folded proteins

The ubiquitin -proteasome system plays a significant role in the " Quality Assurance" intracellularly produced proteins. Proteins should be during and after their production properly folded to make them work. For some proteins, the folding is complex and prone to errors, as in the case of the chloride ion channel in CFTR epithelial cells, in which up to 60-80 % of the proteins produced are folded incorrectly. These misfolded proteins are bound by so-called molecular chaperones, enzymes that can promote proper folding of the protein under certain circumstances. In a misfolding " MIGHT EXPLODE ", the formation of a protein chaperone ubiquitin E3 ligase complex was observed that the misfolded protein poly- ubiquitinated and thus allows the degradation by the proteasome. In this way it is ensured that affect structurally degenerate proteins ( cytosolic as well as membrane - associated) not the cell biology.

But occurs in the case of the ion channel CFTR in the coding DNA a mutation in a mutation of the phenylalanine at position 508 ( F508 ) is reflected, this leads to poly - ubiquitination and the premature breakdown of all produced CFTR proteins. The result is the clinical picture of cystic fibrosis. Proper function of the mutated ion channel is in principle not excluded. This example shows that the actually positive acting, strict control system of the ubiquitin - mediated degradation of the structurally incorrect proteins can also have negative effects on the organism.

Regulation of the transcription

The first step of protein synthesis is the transcription. In this case, DNA is transcribed on an enzyme, RNA polymerase, to RNA. For the start of transcription polymerase various transcription factors are required to DNA. The accessibility of DNA to transcription factors and the polymerase can be permanently attached to the DNA of protein complexes, histones, regulated. Histones, which are "wrapped " of DNA, nucleosomes are called.

In yeast, the ubiquitin linking protein Rad6 was discovered that regulates the transcription of ARG1 ( argininosuccinate synthase Gen1 ). In the absence of the Rad6 transcription factors and the polymerase can bind to the promoter ( a DNA regulatory sequence) of ARG1 gene and the transcription start. In the presence of Rad6 this linked a ubiquitin molecule to the lysine K123 of histone subunit H2B. This leads to modifications of histone H3 in the neighboring nucleosome: The histone H3 is methylated at lysine residues K4 and K49. As a result, the promoter is immobilized, so that no transcription factors can bind. By this gene - silencing the gene ARG1 is no longer expressed and the enzyme argininosuccinate synthase no longer produced in the cell.

In addition, the histone H2A of Drosophila was the first ubiquitinated protein has been described. In mammals, the histone H2A and H2B Ubiquitinierungszustand for the first marker for transcriptionally active chromatin, the total of the DNA and its associated proteins was.

Ubiquitin as part of the signal transduction

Ubiquitin is also involved in the intracellular signal - routing of external stimuli, such as the NF -kB pathway (german nuclear factor kappa B). It is set by the signaling molecule tumor necrosis factor (TNF ) are activated. TNF binds to the TNF receptor of the cell membrane, the E3 ligase TRAF2 recruited to the intracellular portion of the receptor by its conformation. These poly- ubiquitinated itself and the protein RIP on K63 connections. By ubiquitinated proteins RIP and TRAF2 are different kinases phosphorylating enzymes activated. The Iκ kinase β ultimately phosphorylates the protein IkB. This now frees up the previously bound and inactive NF -kB. NF -kB migrates into the nucleus and activate the transcription of specific genes. IkB, however, is degraded via K48 poly- ubiquitination and the proteasome.

Other examples of ubiquitylations

  • After the end of mitosis involved in the cell cycle Cyclin is degraded by ubiquitination.
  • In the HIV anti- viral enzymes of the cell ( ABOBEC3G ) by a viral HIV protein ( Vif ) can be attached. Vif is able to bind the same parts of the ubiquitination machinery. Vif is ubiquitinated and thus degraded with APOBEC3G, thereby increasing the efficiency of HIV infection is enhanced.
  • Increased mono- ubiquitination in the differentiation of multipotent stem cells.

Diseases

Angelman Syndrome is a neurological disorder that among other things, to expressed by a slowed cognitive and motor development. The most common genetic defect here is a 4 million ( MBp ) base pair deletion in the maternal chromosome 15 locus q11 - 13th However, this region is only in the hippocampus and cerebellum active and encodes among others for the E3 ubiquitin ligase E6 -AP. Mice lacking this ligase, develop learning deficits, for example in the conditioning of fear. The longer- term neural plasticity of the mice no longer exists. These deficits correlate partly with the impairments of patients with Angelman syndrome.

  • Dominantly inherited disease in the von Hippel- Lindau a mutation in the gene of the VHL ubiquitin leads to an accumulation of the transcription factor hypoxia-induced factor (HIF ) and tumorigenesis.
  • Parkin mutations of ubiquitin has been shown in certain forms of Parkinson's disease.
  • Mutations Cullin7 E3 ubiquitin ligase were identified as the cause of autosomal recessive disorder growth 3-M syndrome.
Human Genome Organisation Unicellular organism Reaction mechanism Chemical reaction Adenosine triphosphate Adenosine monophosphate Lysosome Ribosome Protein folding Chloride channel Epithelium Open reading frame RNA-Polymerase Promoter (genetics) Protein subunit Gene silencing Drosophila melanogaster Hypoxia-inducible factors Recessive trait Science (journal) Digital Object Identifier Cell (journal) Oncogene (journal)
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