Identification of cysteine residues alkylated with 3-bromopropylamine by protein sequence analysis.

A new reagent for the routine identification of cysteine residues during protein sequencing is described. This method employs 3-bromopropylamine to alkylate cysteines in proteins after reduction with dithiothreitol. Upon sequencing of the protein on an Applied Biosystems 477A protein sequencer, the aminopropylcysteine residue yields a phenylthiohydantoin (PTH) derivative which elutes reproducibly at a unique position immediately after PTH-leucine; baseline resolution is achieved without modification of the PTH analyzer gradient. Unlike PTH-pyridylethylcysteine, the relative elution position of the aminopropylcysteine PTH derivative is not affected by changes in the ionic strength of the analyzer solvents. In addition, the previewing of the next amino acid which is observed in proteins modified with 4-vinylpyridine does not occur in aminopropylated proteins. Preparation of alkylated proteins for electroblotting onto polyvinylidene difluoride (PVDF) membranes and methods for desalting alkylated proteins immobilized on precoated glass fiber filters or PVDF membranes are also described.