An electrophoretic mobility shift assay (EMSA) or mobility shift electrophoresis, also referred as a gel shift assay, gel mobility shift assay, band shift assay, or gel retardation assay, is a common affinity electrophoresis technique used to study protein–DNA or protein–RNA interactions.
A mobility shift assay
is electrophoretic separation of a protein–DNA or
protein–RNA mixture on a polyacrylamide or agarose
gel for a short period (about A. 5-2 hr for a 15- to
20-cm gel). The speed at which different
molecules (and combinations thereof) move through
the gel is determined by their size and charge, and
to a lesser extent, their shape (see gel
electrophoresis). The control lane (DNA probe
without protein present) will contain a single band
corresponding to the unbound DNA or RNA fragment.
However, assuming that the protein is capable of
binding to the fragment, the lane with protein
present will contain another band that represents
the larger, less mobile complex of nucleic acid
probe bound to protein which is 'shifted' up on the
gel (since it has moved more slowly).
Under the correct experimental conditions, the interaction between the DNA (or RNA) and protein is stabilized and the ratio of bound to unbound nucleic acid on the gel reflects the fraction of free and bound probe molecules as the binding reaction enters the gel. This stability is in part due to a "caging effect", in that the protein, surrounded by the gel matrix, is unable to diffuse away from the probe before they recombine. If the starting concentrations of protein and probe are known, and if the stoichiometry of the complex is known, the apparent affinity of the protein for the nucleic acid sequence may be determined. Unless the complex is very long lived under gel conditions, or dissociation during electrophoresis is taken into account, the number derived is an apparent Kd. If the protein concentration is not known but the complex stoichiometry is, the protein concentration can be determined by increasing the concentration of DNA probe until further increments do not increase the fraction of protein bound. By comparison with a set of standard dilutions of free probe run on the same gel, the number of moles of protein can be calculated.
An antibody that recognizes
the protein can be added to this mixture to create
an even larger complex with a greater shift. This
method is referred to as a supershift assay, and is
used to unambiguously identify a protein present in
the protein – nucleic acid complex.
Often, an extra lane is run with a competitor oligonucleotide to determine the most favorable binding sequence for the binding protein. The use of different oligonucleotides of defined sequence allows the identification of the precise binding site by competition (not shown in diagram). Variants of the competition assay are useful for measuring the specificity of binding and for measurement of association and dissociation kinetics.
Once DNA-protein binding is determined in vitro, a number of in silico algorithms can narrow the search for identification of the transcription factor. Consensus sequence oligonucleotides for the transcription factor of interest will be able to compete for the binding, eliminating the shifted band, and must be confirmed by supershift. If the predicted consensus sequence fails to compete for binding, identification of the transcription factor may be aided by Multiplexed Competitor EMSA (MC-EMSA), whereby large sets of consensus sequences are multiplexed in each reaction, and where one set competes for binding, the individual consensus sequences from this set are run in a further reaction.
For visualization purposes, the nucleic acid fragment is usually labeled with a radioactive, fluorescent or biotin label. Standard ethidium bromide staining is less sensitive than these methods and can lack the sensitivity to detect the nucleic acid if small amounts of nucleic acid or single-stranded nucleic acid(s) are used in these experiments. When using a biotin label, streptavidin conjugated to an enzyme such as horseradish peroxidase is used to detect the DNA fragment (Non-radioactive EMSA review). While isotopic DNA labeling has little or no effect on protein binding affinity, use of non-isotopic labels including flurophores or biotin can alter the affinity and/or stoichiometry of the protein interaction of interest. Competition between fluorophore- or biotin-labeled probe and unlabeled DNA of the same sequence can be used to determine whether the label alters binding affinity or stoichiometry.
1. Garner MM, Revzin A (July 1981). Nucleic
Acids Res. 9 (13): 3047–60.
doi:10.1093/nar/9.13.3047. PMC 327330. PMID
2. Fried M, Crothers DM (December 1981). Nucleic Acids Res. 9 (23): 6505–25. doi:10.1093/nar/9.23.6505. PMC 327619. PMID 6275366.
3. Ausubel, Frederick M. (1994). Current Protocols in molecular biology. Chichester: John Wiley & Sons. pp. 12.2.1–11. ISBN 0-471-50337-1.
4. Fried MG, Liu G. (1994). Nucleic Acids Research 22(23): 5054-5059. doi: 10.1093/nar/22.23.5054. PMID 7800499.
Fried MG (1989). 10, 366-376. PMID 2670548.
5. Smith AJ, Humphries SE (January 2009). J. Mol. Biol. 385 (3): 714–7. doi:10.1016/j.jmb.2008.11.035. PMID 19059416.