9 Internalization and activation status of VEGFR2 in response to VEGF165a. were expressed as a percentage of responses obtained using 30?nM VEGF165a-TMR (100%) or vehicle (0%). Data were fitted using non-linear least squares regression (variable slope) using GraphPad Prism. All data were expressed as mean??S.E.M and pooled from 8 independent experiments. For 120?min time course data of VEGF165a-TMR internalization, VEGFR2 activation (measured by anti p1175 or pY1214 labeling) or VEGFR2-Halo internalization, data were normalised as a percentage of peak responses measured at 20?min stimulation (100%) or with no agonist stimulation (0%) respectively. Data for VEGF165a-TMR internalization were fitted to a mono exponential association function: =?All data obtained from the NFAT luciferase reporter gene assays were normalised to 10?nM VEGF165a responses and fitted with non linear regression using the equation as described previously . Statistical analysis used unpaired All data were expressed as percentage fold increases in cell count following VEGF165a or VEGF165a-TMR treatment when normalised to vehicle treatment alone (100%). All data were expressed as mean??S.E.M. Statistical analysis using one way ANOVA was performed to compare vehicle with ligand treatments (P?0.001). The n values in the text show the number of individual repeat experiments. 3.?Results 3.1. Synthesis of an active fluorescent variant of the VEGF165a homodimer labeled on a single N-terminal cysteine Synthesis and purification of a fluorescent variant of VEGF165a labeled on a single amino acid was achieved as depicted in Fig.1a. Briefly, VEGF165a was genetically fused to an N terminal HaloTag via a short amino acid linker made up of a modified TEV recognition site (EDLYFQC), which upon proteolytic cleavage releases VEGF165a with an N terminal cysteine residue (cys-VEGF165a). The secreted fusion protein expressed in HEK293T cells was covalently captured onto HaloLink beads . Cys-VEGF165a was released from the beads by proteolytic cleavage using HaloTEV protease, while HaloTag and HaloTEV remained permanently attached to the beads eliminating the need for post cleavage removal. Proteolytic cleavage in the presence of TMR fluorophore coupled to 2-cyanobenzothiazole (6-TMR-PEG-CBT)  enabled site specific labeling of the released cys-VEGF165a via condensation JI051 of 6-TMR-PEG-CBT JI051 and the uncovered N terminal cysteine . This purification and labeling reaction were performed in a physiological buffer under reducing conditions (100?M tris(2-carboxyethyl)phosphine; TCEP). The purified and labeled VEGF165a (VEGF165a-TMR) was collected and dialyzed to allow final formation of the di-sulphide linked anti-parallel VEGF165a homodimer under non-reducing conditions. Open in a separate window Fig. 1 Synthesis and characterisation of purified VEGF165a-TMR. (a) Synthetic strategy for purification and labeling of VEGF165a-TMR. (b) Fluorescent SDS-PAGE analysis (of VEGF165a-TMR (Eex?=?532?nm; Eem?=?580?nm) in the presence or absence of 100?mM DTT and with or without deglycosylation by PNGase. (c) Influence of bovine serum albumin (0.1% BSA) and 10?mM DTT on VEGF165a-TMR concentrations measured using fluorescence correlation spectroscopy (FCS). Data are from 3 impartial experiments and expressed as mean??SEM. (d) Stimulation of NFAT luciferase production by HEK293T cells stably expressing VEGFR2 by VEGF165a (R&D Systems; closed circles), VEGF165a prepared identically to the fluorescent analogue (open circles) or fluorescent VEGF165a-TMR (red circles). Values represent mean??SEM from 4 independent experiments from which quadruplicate determinations were made. Data are expressed as a percentage of the response to 10?nM VEGF165a (R&D Systems) obtained in each individual experiment. (e) Effect of Tetracosactide Acetate VEGF165a and VEGFR165a-TMR on proliferation of human umbilical endothelial cells (HUVECs). Following stimulation with JI051 VEGF165a or VEGF165a-TMR (3 or 30?nM) for 24 or 48?h, HUVECs were fixed using 3% PFA/PBS and the nuclei stained using “type”:”entrez-nucleotide”,”attrs”:”text”:”H33342″,”term_id”:”978759″,”term_text”:”H33342″H33342 (2?mg/ml). Cells were imaged using a IX Micro widefield platereader at 4 magnification and nuclei were counted using a granularity algorithm (MetaXpress, Molecular Devices). Data are presented as fold increases in proliferation compared to vehicle treatment (mean??S.E.M) and are pooled from 5 individual experiments. One way ANOVA was used to determine the statistical significance of ligand treatment when compared to vehicle only for both.