Allosteric Activation of Human Glucokinase by Free Polyubiquitin Chains and Its Ubiquitin-dependent Cotranslational Proteasomal Degradation

Lise Bjørkhaug,Oddmund Søvik,Torgeir Flatmark,Paål Rasmus Njølstad,Janne Molnes

doi:10.1074/jbc.m700517200

Lise Bjørkhaug, Oddmund Søvik + Show 3 more

Open Access

https://doi.org/10.1074/jbc.m700517200

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Abstract

Human glucokinase (hGK) is a monomeric enzyme highly regulated in pancreatic beta-cells (isoform 1) and hepatocytes (isoforms 2 and 3). Although certain cellular proteins are known to either stimulate or inhibit its activity, little is known about post-translational modifications of this enzyme and their possible regulatory functions. In this study, we have identified isoforms 1 and 2 of hGK as novel substrates for the ubiquitin-conjugating enzyme system of the rabbit reticulocyte lysate. Both isoforms were polyubiquitinated on at least two lysine residues, and mutation analysis indicated that multiple lysine residues functioned as redundant acceptor sites. Deletion of its C-terminal alpha-helix, as part of a ubiquitin-interacting motif, affected the polyubiquitination at one of the sites and resulted in a completely inactive enzyme. Evidence is presented that poly/multiubiquitination of hGK in vitro serves as a signal for proteasomal degradation of the newly synthesized protein. Moreover, the recombinant hGK was found to interact with and to be allosterically activated up to approximately 1.4-fold by purified free pentaubiquitin chains at approximately 100 nm (with an apparent EC(50) of 93 nm), and possibly also by unidentified polyubiquitinated proteins assigned to their equilibrium binding to the ubiquitin-interacting motif site. The affinity of pentaubiquitin binding to hGK is regulated by the ligand (d-glucose)-dependent conformational state of the site. Both ubiquitination of hGK and its activation by polyubiquitin chains potentially represent physiological regulatory mechanisms for glucokinase-dependent insulin secretion in pancreatic beta-cells.

Highlights

Besides its expression in hepatocytes as a cytoplasmic and nuclear enzyme, where its translocation and activity is regulated by the glucokinase regulatory protein and the metabolic state of the cell [1], human GK is expressed in pancreatic ␤-cells mainly as a soluble cytoplasmic enzyme
Degradation of Newly Synthesized Human glucokinase (hGK) in a Proteasome- and ATP-enriched Reticulocyte Lysate System—Degradation of newly synthesized and partially ubiquitinated [35S]Met-labeled His6-hGK was assayed at 30 °C, with a reaction time of 30 min, in an incubation mixture containing in a final volume of 25 ␮l: 1 ␮g of plasmid DNA, 0.5 ␮l [35S]Met (10 ␮Ci), 5 mM ATP, 20 mM dithiothreitol, 10 ␮M Ub, 5 mM MgCl2, E1 (200 nM) and UbcH1-E2 (1 ␮M), 10 mM phosphocreatine, 0.2 mg/ml creatine phosphokinase, and 20 ␮l of rabbit reticulocyte lysate (RRL) preincubated in the absence or presence of 100 ␮M proteasome inhibitor MG132 and 2 ␮M Ub aldehyde
In Vitro Synthesis and Posttranslational Ubiquitination of hGK—As seen from Fig. 2, lane 1, [35S]methionine (Met)-labeled His6-hGK synthesized in the coupled in vitro transcription-translation system of the RRL is a substrate for the Ubconjugating enzyme system of the lysate