This article describes a method for isolating target signal transducer proteins from insect cell lysates using cation exchange chromatography. The process involves optimizing buffer conditions to enhance protein solubility and binding efficiency.
Cation exchange chromatography facilitates protein separation from mixtures based on their net surface charge.
For target signal transducer protein isolation from insect cell protein lysate, obtain a dialyzed target protein lysate containing other contaminating proteins. Add sodium chloride-containing buffer to reduce lysate viscosity and improve protein solubility, which helps prevent chromatographic column clogging during run.
Additionally, buffer pH below the target protein's isoelectric point, pI, at which the protein has zero net charge, facilitates binding of excess hydrogen ions to negative carboxylate groups, making the target protein positively charged.
Assemble the chromatographic column containing crosslinked agarose beads coupled to negatively-charged groups. Add binding buffer to equilibrate the column and maximize column-protein interactions. Load the diluted lysate onto the equilibrated column.
More positively-charged target proteins interact and bind tightly to the negatively-charged groups of the column than less positively-charged contaminating proteins.
Wash the column with buffer to remove unbound negatively charged contaminating proteins. Subsequently, pass elution buffer, while gradually increasing its ionic strength.
At low ionic strength, there is minimal competition between the positive ions and proteins for the negative groups in the column, eluting weakly bound, less positively-charged contaminating proteins. However, as the ionic strength is increased, salt ions compete with tightly bound target signal transducer proteins and reduce interactions between the proteins and the column, eluting them as a purified fraction.
Prepare a 20-milliliter cation exchange column by washing it with three column volumes of Buffer G, then with three column volumes of Buffer F. Next, dilute 20 milliliters of the dialyzed sample to a final sodium chloride concentration of 100 millimolar, by adding 20 milliliters of Buffer E, and apply the sample to the exchange column.
It is critical to dilute the small amount of the sample instead of all at once, and the diluted samples should be applied to the column immediately after dilution to limit precipitation of the protein.
Continue to load the column with freshly diluted sample as the previous dilution nears the end of the load. Then, wash the column to baseline absorbance 280 with Buffer F, which typically required three column volumes.
Elute the protein from the column with a 400-milliliter gradient from Buffer F to 65% Buffer G, collecting the eluate in 6-milliliter fractions. Once the gradient is complete, continue washing the column for an additional 1.5 column volumes of 65% Buffer G.
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