Preparation of Cell Lysate
1. Wash adherent cells twice in the dish or flask with ice-cold PBS and drain off PBS. Wash non-adherent cells in PBS and centrifuge at 800 to 1000 rpm in a table-top centrifuge for 5 minutes to pellet the cells.
2. Add ice-cold modified RIPA buffer to cells (1 ml per 107 cells/100 mm dish/150 cm2 flask; 0.5 ml per 5 x 106 cells/60 mm dish/75 cm2 flask).
3. Scrape adherent cells off the dish or flask with either a rubber policeman or a plastic cell scraper that has been cooled in ice-cold distilled water. Transfer the cell suspension into a centrifuge tube. Gently rock the suspension on either a rocker or an orbital shaker in the cold room for 15 minutes to lyse cells.
4. Centrifuge the lysate at 14,000 x g in a precooled centrifuge for 15 minutes. Immediately transfer the supernatant to a fresh centrifuge tube and discard the pellet.
5. Dilute the cell lysate at least 1 : 10 before determining the protein concentration because of the interference of the detergents in the lysis buffer with the Coomassie-based reagent. At this step, the sample can be divided into aliquots and stored at -20íC for up to a month.
TIP: When working with large volumes of non-adherent cells, the cells may not be cooled quickly enough to maintain the activity of the protein being studied. In this case, pour the cell suspension into a mixture of an equal mass of 2 x PBS and ice, then collect the cells by centrifugation and perform the lysis as described above.