A Na 2 HPO 4 14.6 g

KH2PO4 5.4 g 15 NH4Cl 1.0 g B Glucose 4.0 g Thiamine hydrochloride 20 mg 5 g/L MnSO4 0.5 ml 37.5 g/L CaCl2 0.5 ml 1 g/L FeCl2 0.5 ml

Table 2.13 Recipe for SDS-PAGE resolving and stacking gel: Table shows the differing acrylamide concentration used for resolving gels depending on the expected band sizes and the fixed 4% (w/v) acrylamide for stacking gels.

Reagent Resolving gel (ml)

8% 10% 12%

Protogel acrylamide solution 2.67 3.33 4.0

Resolving buffer 2.6 2.6 2.6

dH2O 4.62 3.96 3.29

TEMED 0.01 0.01 0.01

10% APS 0.1 0.1 0.1

Total Volume 10.0 10.0 10.0

Reagent Stacking gel (ml)

4%

Protogel acrylamide solution 0.65

Stacking buffer 1.25

dH2O 3.05

TEMED 0.005

10% APS 0.025

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2.5 Biophysical procedures 2.5.1 Reagents

PD10 desalting column was from Amersham (Buckinghamshire, UK). Wizard: Cryo™ and Wizard™ Screens I and II were from Emerald BioSystems (Washington, USA), ProPlex™

was from Molecular Dimensions (Suffolk, UK), Crystal Screen was from Hampton Research (California, USA). ITC system was from MircoCal Inc. (Buckinghamshire, UK).

2.5.2 Gel filtration chromatography

The protein eluted from the ion exchange column (Section 2.4.2.2) or from -80°C freezer was passed through a PD10 column which had been equilibrated with gel filtration buffer (Table 2.11). 500 μl of individual proteins were injected onto the gel filtration column (Superdex 75 10/300 GL) which was run at 1 ml/min. The step was repeated to the premix of both proteins in 1:1 followed by varying ratio to assess complex formation.

2.5.3 Isothermal titration calorimetry

All proteins were buffer-exchanged into ITC buffer (Table 2.11) using a PD10 column which had been equilibrated with ITC buffer and concentrated to 0.1 mM (protein A) and 2.1 mM (protein B). All protein samples and buffers used in this experiment were degassed for 5 minutes. ITC cell chamber was washed three times with the degassed buffer before pipetting 300 μl of 0.1 mM protein A into the chamber. Meanwhile, 80 μl of 2.1 mM protein B (or at

least ten-fold from the concentration of protein A) was loaded to the syringe. The injection of sample from the syringe to the cell was performed at 180s time intervals, 3 μl per injection and the cell temperature was set at 25°C.

2.5.4 Nuclear magnetic resonance

Protein in NMR buffer was diluted or concentrated to 0.1-0.5 mM. The sample was prepared for NMR by adding 5% (v/v) D2O to 600 μl of protein sample. Then, the sample was placed

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in a clean standard NMR tube (5 mm diameter) by a long Pasteur pipette. Care was taken not to introduce any air into the NMR tube. All NMR spectra were collected at 25°C using a Bruker Avance 600 or 800 MHz solution-state spectrometer. For the chemical shift perturbation experiments, (1H15N)-HSQC 2D spectra were recorded on 15N-labelled protein alone and titration with different ratios of its unlabelled partners (1:0.25, 1:0.5, 1:0.75, 1:1, 1:1.5, 1:2). For all (1H15N)-HSQC spectra, 2 048 points with a spectral width of 8 012 Hz in the direct dimension and 512 points with a spectral width of 2 200 Hz in the indirect dimension were recorded. The number of scans varied between 16 and 64 during the titration and the relaxation delays were set to 1s. All NMR spectra were processed and analysed using Bruker TopSpin 3.1.

2.5.5 Crystallisation trial

Two sets of 500 μl of OTUB1:UBE2D2 complex were prepared in 10 mg/ml and 20 mg/ml

alongside 10 mg/ml free protein. 80 μl of each condition (Wizard, ProPlex, Crystal and Cryo) were pipetted into the reservoir of the MRC 96-well sitting drop vapour diffusion crystallisation plate (Molecular Dimensions). 0.5 μl of each protein samples were pipetted into drop reservoir followed by 0.5 μl from each condition. Plates were sealed with crystal sealing film and incubated at RT and the other set at 4°C for about two weeks.

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2.6 Cell biology 2.6.1 Reagents

All cell culture reagents were obtained from Invitrogen unless otherwise stated. Plasticware was obtained from Corning Inc. (New York, USA) and chemicals were from Sigma unless otherwise stated.

2.6.2 Cell culture

HeLa cells were cultured in a humidified 5% (w/v) CO2 atmosphere at 37°C in Dulbecco’s Modified Eagle’s Medium (DMEM) supplemented with 10% (v/v) foetal bovine serum, 1%

(v/v) non-essential amino acids (NEAA), and 1% (v/v) penicillin/streptomycin sulphate. Cells were maintained by growing in 75 cm flasks. Every three to four days when the cells reach ~80% confluency, the media was removed and cells were washed with 30°C PBS before being trypsinised with 2 ml trypsin for 2 minutes in 37°C (5% (w/v) CO2) incubator. Cells were then split into 1:5 to 1:10 dilution into a new flask with fresh DMEM.

2.6.3 Cell transfection and fixation

80% confluent cells were trypsinised as above and 0.5x105 cells (counted by hemocytometer) were seeded onto coverslips in 12-well plate containing 1 ml of DMEM. They were incubated at 37°C (5% (w/v) CO2) for 24 hours until cells reached 50%-80% confluency. On the next day, 1.5 µl Genejuice was added to 50 µl DMEM media with no additional supplements and mixtures were vortexed gently and mixed at RT for 5 minutes before adding 0.5 µg of DNA. They were mixed by gentle pipetting and incubated at RT for 15 minutes before being pipetted into 12-well plates in a drop-wise manner to ensure the whole coverslip was covered. They were then mixed by gently rocking at 37°C (5% (w/v) CO2) for 24 hours. Optionally, transfection mixture was removed after 4 hours incubation and replaced with complete DMEM. On the next day, the DMEM was removed and cells were rinsed twice with 1 ml of 30°C PBS before fixing with 300 µl of 4% (v/v)

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paraformaldehyde. The 12-well plates were then placed on a rocker for 15 minutes to gently mix. Coverslips were then rinsed twice with PBS and dH2O and the edges of coverslips were blotted. 10 µl of Vectashield® HardSet™ mounting reagent containing DAPI for nuclear staining was put onto slides and the coverslips were mounted immediately with the cells facing downwards to the Vectashield®. They were left to air-dry for 1 hour before storing in the dark at 4°C. Cells were visualised using an Olympus Model IX81 microscope system. All images were taken with a Hamamatsu C4742-80 camera.

2.7 Bioinformatic method

Four public databases namely the Human Protein Reference Database (HPRD) (Keshava Prasad et al., 2009), Molecular Interaction database (MINT) (Chatr-aryamontri et al., 2007), IntAct (Aranda et al., 2010) and BioGRID (Breitkreutz et al., 2008) were mined by Russell Hyde (University of Liverpool) to provide overlapping yet complementary datasets to detail as many literature-reported interactions as possible. The data were integrated to produce human ‘interactome’ network containing both binary protein-protein interactions observed

and more ambiguous protein interactions derived from co-complexes detection techniques. To analyse or predict an interaction data, computational filters written by Jonathan Woodsmith (University of Liverpool) were utilised. Visualisation of protein-protein interaction data were generated using the open source bioinformatics software platform Cytoscape. All cytoscape figures were generated using Cytoscape version 2.8.2.

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3.1 Introduction

To provide a better understanding of the order and specificity within the human ubiquitination process, it is crucial to perform a detailed analysis of protein interaction preferences within this system. Previous large-scale Y2H screens performed in our lab generated a high-density map of human E2:E3 RING interactions and identified a surprising interaction involving a specific subset of E2 proteins (UBE2Ds and UBE2Es) and a deubiquitinating enzyme (DUB) called OTUB1 (Figure 3.1A) (Markson et al., 2009). This data raised an interesting question as to why OTUB1 should associate with specific E2 proteins. Classically, DUB proteins are well-known for cleaving or trimming ubiquitin chains from the substrates and although DUB proteins had been found to interact with some E3-RING proteins, no association with E2-ubiquitin conjugating enzymes had been observed.

During the course of this project, a large-scale systematic mass-spectrometry analysis of human DUB proteins was performed in which interactions between OTUB1 and UBE2D2 and UBE2N proteins were observed (Figure 3.1B) (Sowa et al., 2009). Functional investigation of these non-canonical DUB:E2 complexes revealed a role in DNA damage response and p53 signalling (Nakada et al., 2010; Sun et al., 2012). Although these reports have addressed two functional roles of E2:OTUB1 complexes, little attention was given to define the molecular mechanism of E2:OTUB1 interactions.