Tandem affinity purification of proteins.

Tandem affinity purification (TAP) is a very efficient method to isolate proteins, protein complexes, or ribonucleoprotein particles from crude extracts. The method depends on the expression of one protein component fused N- or C-terminally to a TAP tag in the organism of interest. The TAP tag is a composite tag consisting of two different epitope domains and a protease cleavage site, and it facilitates the purification of the tagged protein in two consecutive, high-affinity chromatography steps. Combined, the two steps are typically so efficient that a protein complex can be purified virtually to homogeneity without the need for protein overexpression. If the tag does not interfere with protein function, TAP is likely to yield an intact protein complex because all steps of the procedure are carried out under nondenaturing conditions. In this unit, a TAP procedure is detailed which employs a novel epitope combination termed PTP.