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Sample preparation is critical to the success of two-dimensional gel electrophoresis and other analytical methods. Pressure-cycling technology (PCT) uses alternating cycles of high and low pressure to induce cell lysis. Cell suspensions were placed in PULSE Tubes and subjected to alternating cycles of high and low pressure in a Barocycler instrument. each cycle consisted of 20 sec at 35,000 psi followed by 20 sec at ambient pressure. For the bacterium Escherichia coli, PCT extracted 14.2% more total protein than was extracted using a standard bead mill. Image analysis of two-dimensional gels revealed 801 protein spots in the PCT lysate, compared to 760 protein spots in the bead mill lysate.
The bacterium Escherichia coli is a widely used host for protein expression. However, the recovery of recombinant proteins hinges largely on the ability to effectively induce cell lysis. Similarly, the understanding of cellular processes is facilitated only when methods such as two-dimensional gel electrophoresis (2DGE) are capable of isolating without bias the entire protein constituency of such model organisms. An accurate representation of any organism’s entire proteome can be derived only when all cells are efficiently lysed. This necessarily involves the physical disruption of cell walls or membranes, and the chemical means to ensure the solubility of released proteins.
Probe sonicators and bead mills are commonly used for cell lysis. However, both sonication and bead mill oscillation can generate excessive heat, which rapidly accelerates the hydrolysis of urea and the formation of isocyanic acid, resulting in the potential carbamylation of protein amines. In addition, sonicators and bead mills present potential health risks for laboratory workers. Probe sonicators inherently aerosolize potentially pathogenic agents. Moreover, at least two researchers have been infected with West Nile virus when tubes oscillating in a bead mill ruptured.1
Pressure-cycling technology (PCT) uses rapid alternating cycles of high and low pressure to induce cell lysis. Cell suspensions or tissues are placed in specially designed processing containers (PULSE Tubes) and are subjected to alternating cycles of high and low pressure in a pressure-generating instrument (Barocycler Models NEP2017 or NEP3229) developed by Pressure BioSciences (West Bridgewater, MA). Pressure in the PULSE Tube increases to 35,000 psi in less than 3 sec and returns to ambient pressure in less than 1 sec. Maximum and minimum pressures, the time sustained at each pressure level, and the number of cycles are defined using a computer or programmable logic controller interface. The Barocycler instrument reaction chambers are temperature controlled using a peripheral circulating water bath. Safety features in the design of the PCT sample preparation system significantly reduce risk of researcher exposure to pathogens.2
To illustrate PCT performance, 60 mg of lyophilized E. coli K12 were reconstituted in 10 mL of 7 M urea, 2 M thiourea, and 25 mM 3-(4-heptyl) phenyl 3-hydroxypropyl dimethylammonio propanesulfonate (C7BzO).3 Twenty-five microliters of 200 mM tributylphosphine (Sigma-Aldrich, St. Louis, MO) were added, and 1.5 mL of this cell suspension was transferred to each of two PULSE Tubes. The PULSE Tubes were subjected to five pressure cycles in the Barocycler NEP2017 instrument. Each cycle consisted of 20 sec at 35,000 psi followed by 20 sec at ambient pressure. The contents of the two PULSE Tubes were combined and centrifuged at 24,000 relative centrifugal forces (RCF) for 10 min to remove cellular debris.
Alternatively, 1.5 mL of the cell suspension was transferred to each of two 2-mL polypropylene tubes for the bead mill procedure. Sample tubes were loaded into the adapter rack of a Retsch MM 301 mixer mill with tungsten carbide grinding balls provided by the manufacturer (Retsch GmbH, Haan, Germany). The tubes were cycled three times for 1 min at 1800 oscillations per min. After each cycle, the temperature of the samples reached 40°C, requiring that the tubes be removed from the adapter and placed on ice for several minutes between cycles (a potentially precarious practice, since the solubility of urea, thiourea, and the C7BzO detergent is compromised below 18°C). The contents of the two tubes were combined and centrifuged at 24,000 RCF for 10 min to remove cellular debris.
Three milliliters of each lysate were alkylated for 2 h following the addition of 10 mM acrylamide and 40 mM Tris. Proteins were precipitated with 80% acetone at room temperature for 30 min. The flocculent was pelleted by centrifugation at 24,000 RCF for 10 min. Pellets were dissolved in 3 mL of ion-exchanged 7 M urea, 2 M thiourea, and 65 mM CHAPS. Dried, immobilized pH gradient strips, pH range 3–10 (Proteome Systems, Woburn, MA), were hydrated with 0.2 mL of each lysate for 6 h. Isoelectric focusing and 2DGE was performed as described.4 Image analysis was performed using Progenesis Discovery and Editor software (Nonlinear Dynamics, Newcastle Upon Tyne, UK).
PCT extracted 14.2% more total protein from E. coli than was extracted using a bead mill. Image analysis of the 2D gels (Figure 11)) revealed several low-abundance proteins in the PCT lysate that were not detected in the bead mill lysate. A total of 801 protein spots were detected in the PCT lysate, compared to 760 protein spots in the bead mill lysate. The graph in Figure 22 provides numerical data to support the visual impression in Figure 11.. The preponderance of points in the ordinal quadrant shows the improved efficiency of cell lysis by PCT as compared to bead mill oscillation, since the integrated spot volumes measured in 2D gels deviated from the theoretical slope of 1.00. A slope of 1.00 is expected only if the methods are equivalent.
Finally, the temperature in the Barocycler remained constant at 22°C throughout 10–20 pressure cycles, compared to the bead mill or probe sonicator, in which the temperature can exceed 40°C within seconds.
The authors acknowledge the contributions of Alexander Lazarev of Proteome Systems, Inc., and Sunny Tam and Douglas Hinerfeld, of UMMS Proteomics Consortium.