'In gel' cleavage with cyanogen bromide for protein internal sequencing.

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) is a powerful purification technique in protein chemistry research. This procedure is frequently used as a last step in protein purification for sequencing. For proteins which are N-terminal blocked, 'in gel' digestion offers a useful approach for the generation of internal sequence data from proteins purified by SDS-PAGE. In this study, we propose a procedure where proteins purified by this method are chemically cleaved 'in gel' by using CNBr and the resulting peptides are isolated in a second SDS-PAGE. After that, electroblotting is performed onto PVDF membranes and the electroblotted and Coomassie-stained peptide, from each band is then sequenced by Edman degradation. Proteins often have a small number of methionines whose cleavage allows the obtention of long peptides suitable to sequence a good deal of residues. Three standard proteins of different molecular mass have been assayed by this procedure and the 'in situ' cleavage profile compared with direct chemical digestion in a protein solution.