Electrophoresis Tech Note 2778
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02-102 2778 remy tnote.qxd 5/1/02 5:00 PM Page 3 electrophoresis tech note 2778 Focusing Strategy and Influence of Conductivity on Isoelectric Focusing in Immobilized pH Gradients Arnaud Rémy1, Naima Imam-Sghiouar2, Florence Poirier2, and Raymonde run allows evaluation of the entire run and analysis of the Joubert-Caron2 resulting electrical profile. This analysis is useful to determine 1Bio-Rad France, 92430 Marnes la Coquette, France the optimal migration parameters and for troubleshooting. 2Université Paris 13, Laboratoire de Biochimie des Protéines et Protéomique, Although numerous protocols are available in the literature EA 2361, UFR SMBH Léonard de Vinci, 93017 Bobigny, France (Gorg et al. 2000, Laboratoire de Biochimie des Protéines et Correspondence: Dr Arnaud Rémy, Bio-Rad, 3 boulevard Raymond Poincaré, Protéomique server at http://www-smbh.univ-paris13.fr/ 92430 Marnes la Coquette, France, [email protected] lbtp/index.htm) as well as in electrophoresis equipment Summary manufacturers’ instructions, it is difficult to identify the best Sample preparation is a key component of successful two- protocol for a new sample. dimensional (2-D) gel electrophoresis of proteins. No buffer IEF protocols include three major steps: a step at low voltage is universally suitable for the extraction and solubilization of to initiate desalting of the sample; a progressive gradient to protein samples. On one hand, it is necessary to extract the high voltage to mobilize the ions, polypeptides, and proteins; maximum number of proteins and maintain their solubility and a final high-voltage step to complete the electrofocusing. during 2-D electrophoresis. This is commonly done by adding agents like detergents and chaotropic agents to solubilization Establishment of a voltage gradient between two electrodes cocktails. On the other hand, isoelectric focusing (IEF) is most results in mass transfer between them. Three processes successful with samples containing few ionic compounds, contribute to mass transfer: 1) electrical migration, where although the contribution of ions to IEF is not often recognized. charged species move in response to the electrical field; We therefore studied the effects of water quality, DTT, and 2) diffusion, where species move under a chemical potential salt-containing buffers on IEF. We also investigated different gradient (for example a concentration gradient); and factors that affect the applied current during IEF, such as the 3) convection, which occurs as a result of diffusion or method of rehydration with sample and different modes of hydrodynamic transport (Garfin 2000). The movement of voltage ramping. Our initial observations indicate that the total charged matter is observable as an electrical current (I) and number of spots per gel is almost the same under different follows Ohm’s Law, V = I x R, where V is the voltage and R is run conditions. However, variations in spot count were the resistance. The three mass transfer processes, namely the observed outside the 25–100 kD and pl 5–8 ranges under moving current, the equilibrium current, and the convection different running conditions. current, contribute to the observable (net) current. Introduction During electrophoresis in a pH gradient, as in IEF, the moving 2-D gel electrophoresis is a critical technique for proteomic current is due to amphoteric molecules that move in response research, since it is the only method currently available that to the field until they reach their steady-state positions. is capable of simultaneously separating thousands of In addition, ionic species move through the gradient to the polypeptides for quantitative studies (Hochstrasser 1998). oppositely charged electrode. The transport of ionic species One of the most important improvements in 2-D electrophoresis through, and out of, the focusing matrix can take considerable was the introduction of immobilized pH gradients, or IPGs time — a fact that is not often recognized. As each species (Bjellqvist et al. 1982, 1993). Various types of apparatus that reaches an electrode or the point in the IPG at which it is allow direct incorporation of proteins into the IPG during uncharged, its contribution to the net current decreases. rehydration have contributed to higher resolution and higher protein loading capacity. Nevertheless, methods for proper control of electrical parameters during IEF are not often fully appreciated. Recording electrical parameters during an IEF 02-102 2778 remy tnote.qxd 5/1/02 5:00 PM Page 4 The equilibrium current is the current generated by all the IEF was carried out in the PROTEAN IEF cell. The 7 cm species as they diffuse around their equilibrium position, pH 3–10 strips were subjected to the following IEF program: including ions close to the electrode and proteins in the IPG 50 V for 15 min, 250 V for 15 min, linear gradient to 4,000 V as they are converted between their neutral and ionic forms. over 2 hr, and 4,000 V for 2 hr. The longer 17 cm pH 3–10 The equilibrium current is proportional to the concentration of strips were subjected to three different IEF program strategies: all ionized species in the sample. In contrast to the moving 1) a “direct progressive voltage” (50 V for 15 min, 250 V for current that decreases with time at constant voltage, the 15 min, linear gradient to 10,000 V over 5 hr, and 10,000 V equilibrium current is linearly proportional to the voltage. for 5 hr, or, for the protein-containing mixture, for a total of 40 kV-hr ); 2) a “desalting step and progressive voltage” (50 V The convection current may be driven by electroendosmotic for 9 hr, 200 V for 1 hr, linear gradient to 1,000 V over 1 hr, movement, a component of hydrodynamic transport. linear gradient to 10,000 V over 6 hr, and 10,000 V for a total For safety from overheating, a limit of 50 µA per 3 mm wide of 40 kV-hr for the entire run) as described in Joubert-Caron et strip is advised for commercial apparatus. If the current al. (2000); and 3) a “desalting step and rapid voltage” (50 V for reaches 50 µA when the voltage is set to increase linearly, 9 hr, 10,000 V to 40 kV-hr). The current was limited to 50 µA the voltage will then be regulated by the current limit, and the per strip, and the running temperature was 20°C. The strips resulting voltage increase will no longer be linear. Once the were run in duplicate for samples that contained buffers in current decreases below 50 µA, the voltage will again drive the absence of proteins, and in triplicate for in-gel protein the linear progression of the gradient. One of the major incorporation. For protein-containing samples, the strips were problems encountered by novices to 2-D electrophoresis is stored at -20°C until used for the second dimension. the inability to reach the voltage they want. Connection of a PROTEAN IEF Cell to a Computer We evaluated the contribution of various compounds to To monitor electrical properties during IEF, a modem cable the electrical field. We show how recording the electrical was connected from the RS-232 port of the PROTEAN IEF parameters helps to identify artifacts, allowing adjustment of cell to the serial port of a PC. The HyperTerminal software the profile and the duration of each IEF step, and adaptation was opened from the Start menu of the Windows 95 of the protocol to the sample and the buffers used. operating system. The setup configuration was 9,600 bits/sec, 8 data bits, no parity, stop bit at 1, material control flow. Methods The PROTEAN IEF cell exported run data every 5 min to First-Dimension Electrophoresis HyperTerminal software. At the end of the run, the data were All reagents and materials were obtained from Bio-Rad unless entered into an Excel spreadsheet for plotting. otherwise indicated. The loading volumes were 125 µl and 300 µl for ReadyStrip™ 7 cm and 17 cm IPG strips, respectively. Second-Dimension Electrophoresis Passive rehydration was carried out in the PROTEAN® IEF cell Prior to SDS-PAGE, IPG strips were equilibrated as previously at 20°C. The rehydration step was 2 hr long for test samples described in Joubert-Caron et al. (2000). They were loaded on ® that contained buffers in the absence of proteins, and 6–9 hr PROTEAN II Ready Gel 8–16%T precast gels and run for 1 hr for in-gel protein incorporation. at 40 V followed by 15 hr at 150 V in a PROTEAN II XL cell. The gels were silver stained and one gel per condition was ™ Three different premade reagents were tested: ReadyPrep scanned with a GS-700 scanner and quantitated as described reagent 1, reagent 2, and reagent 3.* For protein in-gel by Joubert-Caron et al. (1999) and Poirier et al. (2001) with incorporation, the salt-free rehydration solution was 7 M urea, Melanie 3 software (Genebio, Geneva, Switzerland). 2 M thiourea, 4% (w/v) CHAPS, 0.24% (v/v) Triton X-100, 0.2% (w/v) Bio-Lyte 3/10. Rehydration solution (280 µl) was mixed with 20 µl (100 µg) of soluble proteins extracted from the lymphoblastoid cell line PRI as previously described in Joubert-Caron et al. (2000). To evaluate the effect of DTT on the current, lyophilized reagent 2 was reconstituted with deionized water and with either 10 mM or 20 mM DTT. * Reagent 1 contains 40 mM Tris. Reagent 2 contains 40 mM Tris, 8 M urea, 4% (w/v) CHAPS, and 0.2% (w/v) Bio-Lyte® 3/10 ampholyte. Reagent 3 contains 40 mM Tris, 5 M urea, 2 M thiourea, 2% (w/v) CHAPS, 2% (w/v) sulfobetaine (SB 3-10), and 0.2% (w/v) Bio-Lyte 3/10.