ISG15‐dependent Regulation

Abstract

Introduction and OverviewCells regulate their short-term responses to internal and external signals from the environment through a repertoire of post-translational modifications that alter protein function. This regulatory strategy is ancient in origin, remarkably conserved across phyla, and evolutionarily robust -allowing novel biological applications to arise in response to new challenges. The emergence of eukaryotes with their more complex organizational demands is marked by the appearance in the genomic record of new regulatory strategies involving post-translational modifications that, for the first time, involved low-molecular-weight proteins as modifying groups rather than small inorganic/organic molecules. Ubiquitin represents the first example of this new class of post-translational modifying proteins identified [1-3]. Based on sequence and structural conservation, the bacterial molybdopterin cofactor synthase complex presumably served as the evolutionary template from which ubiquitin and its essential activating enzyme diverged [4][5][6]. Functional aspects of ubiquitin-dependent regulation must have been established relatively soon after radiance of eukaryotes since the polypeptide and most components of the requisite ligation pathways are remarkably conserved across phyla [7]; however, the absolute conservation of the ubiquitin sequence among higher eukaryotes suggests that the polypeptide has continued to acquire new roles, evidence of which is seen in the phylogeny of the E2/Ubc superfamily [8].The regulatory advantage of this strategy cannot be over-emphasized. Because ubiquitin has a larger and more varied water-accessible surface than smallmolecule modifiers, ubiquitin possesses a greater inherent information content. Moreover, the driving forces of gene speciation and natural selection can be exploited to adapt and mould this signalling molecule in ways not possible with immutable smaller post-translational modifying groups such as phosphate. In practical terms these advantages allow ubiquitin to serve as a reversible transposable binding element to alter target protein structure and/or target protein ligand interactions. Ubiquitin molecules can also be linked together to form repeating chains in order to amplify the ubiquitin signal and provide additional diversity. Because 5.3 Structure and Properties of the ISG15 Protein 105

ISG15 disrupts cytoskeletal architecture and promotes motility in human breast cancer cells

“…8 UbcH8 is an ISG15-specific E2 conjugating enzyme that forms an obligate thiolester catalyzed by UbE1L. 9,10 Although several E3s have been identified as possible ISG15 E3 ligases, the major E3 for ISG15 appears to be HERC5. 11 However, the incomplete loss of the ISG15 conjugates in HERC5-ablated cells suggests that other ISG15 E3 ligases may contribute to the ISG15 conjugation in vivo.…”

Measuring Rates of Ubiquitin Chain Formation as a Functional Readout of Ligase Activity

“…Such similarities in E2 structure and E2–E3 interactions allow functionally irrelevant binding interactions to be favored at sufficiently high concentrations of the E2-ubiquitin thiolester, leading to ambiguous conclusions regarding the identity of the cognate E2 for the ligase. A particularly good example of this problem is the ability of UbcH8, an ISG15-specific E2 (11, 12), to support selected ubiquitin conjugation reactions at micromolar concentrations that exceeds its intracellular concentration following interferon induction (13–17). A related problem confounding the determination of the cog-nate E2 relates to the assay conditions with respect to incubation time and E3 concentration since prolonged incubation times and high concentrations of ligase can accentuate trace activities arising from low E2-ubiquitin thiolester binding affinity and/or k cat for noncognate E2 species.…”