Closed c-Src , 1:10,000 for a-tubulin. The membrane was then washed four times over 30 min with PBS-T before probing with horseradish peroxidase-conjugated secondary antibodies for 1 h at room temperature. The membrane was washed four times over 30 min with PBS-T. The protein bands were visualized using enhanced chemiluminence according to manufacturer’s instruction and digital acquisition by a LAS-3000 21560248 imager. Analytical ultracentrifugation Transfected cells were harvested by scraping off the plate in 5 mL DMEM and placed into a tube. Cells were pelleted and pellet was resuspended on ice in 1 mL ice cold native lysis buffer Triton X-100, Complete-EDTA-free protease inhibitor, 0.2% sodium fluoride, 0.2% sodium orthovanadate, 20 U/mL benzonase, 1 mM PMSF). Cells were mechanically lysed on ice by extrusion through an 27 Gauge needle 20 times. 5 M NaCl was added to a final concentration of 150 mM. Lysate was maintained on ice until analytical ultracentrifugation. Total protein concentration was measured using a BCA assay and adjusted to 0.5 mg/mL with NLB. Lysate was loaded into Charcoal-Epon 12 mm thick Velocity quartz-window centerpieces, prechilled at 4uC overnight, and centerpieces placed into a prechilled AnTi-8 rotor also prechilled at 4uC overnight. Samples were analyzed with an XL-A analytical ultracentrifuge equipped with a fluorescence detection module and 488 nm laser at 10uC. Continuous radial fluorescence scans were MedChemExpress 139504-50-0 collected at a rotor speed of 3,000 rpm for 2 h. The rotor speed was increased to 40,000 rpm and a further 200 scans were collected. The sedimenting boundaries were fitted to a model describing the sedimentation of a distribution of sedimentation coefficients c, which were converted to mass units, c, using the program SEDFIT. Data were fitted using a regularization parameter of F = 0.95, TI noise correction and partial specific volume of 0.73 g/mL. The frictional ratios and meniscus position were floated. Buffer viscosity was estimated as 0.013336 Poise, and density 1.00665 g/mL based on buffer composition using SEDNTERP. Results First we probed the basal levels of Y416 phosphorylation of cSrc transfected into AD293 cells by Western Blot. As part of our 
experiments, we included c-Src fused to the GFP derivative Emerald. Fusion of c-Src to GFP with a few spacer residues has been used to study c-Src cell biology without disrupting key functional properties of c-Src. Wild-type c-Src and mutants that constitutively adopt an open, active conformation or Y416 Phosphorylation in Closed c-Src a closed, repressed conformation were investigated alone and in fusion to Emerald. After first standardizing the amount of lysate to c-Src levels, blots were analyzed with two different antibodies for c-Src phospho-Y416. Both antibodies bound specifically to all forms of c-Src as evidenced by no immunoreactivity to a negative control of cells transfected with b-galactosidase. The extent of reactivity of the antibodies was qualitatively greater for the open conformation than the closed, and the wild-type was intermediate. The pattern of phospho-Y416 staining was 7906496 similar in the context of the Emerald tag and without the tag, suggesting that the Emerald does not perturb the fundamental regulatory mechanisms of c-Src. A notable feature of the data was that the repressed, closed conformation of c-Src had a substantial capacity to be phosphorylated at Y416. To further investigate the underlying kinase activity of c-Src under our conditions
