Protein A
Protein A is the name of a protein found on certain bacterial cell walls. The protein has a binding site for Immunoglobulin G (IgG). This ability to bind IgG has been protein A a useful tool for research laboratories. Protein A has many uses. If conjugated to a detectable marker, such as a fluorescent marker or an enzyme, it can be used to detect antibodies. Protein A can also be used to purify total IgG from a solution. Also it can be used for immunoprecipitation as a way to purify a molecule of interest using a solid support system.
Magnetic beads
Magnetic beads are magnetic beads in the micrometer range. The beads are used in a variety to isolate many types of molecules, proteins, cells, etc. The beads are pre-conjugated and/or placed in a solution of carefully optimized pH and ion composition to allow them to bind to various molecules. Once a molecule is bound to the bead, the whole solution is placed on a magnetic rack of some kind. The magnet holds the molecule of interest-bound magnetic beads to the vessel they are in, so that the solution and everything else in it can be removed. There are usually wash steps in which a wash buffer is placed in the vessel while on the magnetic rack. Finally when the elution buffer is placed in the vessel, it can be removed from the magnetic rack so that the molecules can be released into the solution.
Magnetic beads protein A
Magnetic beads that are pre-conjugated to protein A are used for immunoprecipitation. Typically, the protein A on the bead is first allowed to bind with antibodies against an antigen of interest. Then it is ready to bind the antigen of interest from a solution. The final step will be eluting the antigen. To elute the antigen you will need an elution buffer that can weaken the bond between the antigen and antigen (and likely weaken the antibody-protein A bond as well). This may be a more gentle buffer at low pH, or for a stronger buffer you can make a denaturing buffer. You will also need to consider which buffers to use for binding and washing, or you might be doing the precipitation in a lysis buffer from a previous step.
Considerations and Benefits of magnetic beads protein A
Magnetic beads protein A are a useful way to do immunoprecipitation due to their ease of use. Magnetic beads are simple to use by just placing samples on and off a magnetic rack. The magnetic rack creates just the right amount of force to ensure that your magnetic beads stay pressed against the vessel. This lowers the amount of human error introduced into an experiment and makes experiments more efficient. There are even kits available that contain all your necessary buffers for doing such a procedure. Another similar method is using protein G magnetic beads, which also binds immunoglobulins but has slightly different subclass specificities. In situations where one is using protein A or G for binding antibodies, it is useful to distinguish which subclasses are most important and choose which protein to use to bind them.
Some examples of commercially available magnetic beads are Dynabeads protein A and Protein G (Thermofisher), Sera-Mag™ SpeedBeads™ Protein A/g particles (Cytiva) and/or PureProteome Protein A/G (MerckMillipore), but the list of magnetic beads providers offering products for protein immunoprecipitation and purification grows everyday.
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