Comparison of two techniques for protein isolation and radioiodination by tryptic peptide mapping

Isolation of specific proteins from multicomponent systems (e.g., avian and mammalian retroviruses) is best achieved by SDS-polyacrylamide gel electrophoresis (SDS-PAGE), agarose gel filtration in 6 M guanidine hydrochloride (GuHCl), isoelectric focusing, and immunoaffinity chromatography. Using these techniques, purified proteins are prepared which often retain structural integrity and, in many cases, even biological and immunological reactivity, thereby permitting their further analysis in a variety of biochemical assays (l,Z). Twodimensional peptide mapping is a rapid, convenient method for direct structural comparison of such purified polypeptides and can be used to establish the identity of two independently isolated proteins or to verify a precursor-product relationship between related but nonidentical protein species (l-7). Elder er ul. (3) describe a method for radioiodination of fixed proteins within sodium dodecyl sulfate-polyacrylamide gels without elution of the proteins from the gel, followed by two-dimensional peptide mapping. This procedure offers a means of comparing the structure of individual proteins when preparative recovery of each component is not practical. In this communication we report that proteins isolated in a one-