Evidence for the Presence of Only One Cysteine Residue in the D-type Low Molecular Weight Subunits of Wheat Glutenin

D-type low molecular weight subunits of bread wheat glutenin have ω-gliadin type N-terminal amino acid sequences, but are incorporated into the glutenin polymers because of the presence of cysteine residues. In order to determine the number and position of cysteine residues, ID-encoded D-type low molecular weight subunits of wheat glutenin were purified from the cv. Chinese Spring using a procedure that allowed high recovery. Comparison of the molecular weights of alkylated and unalkylated subunits by MALDI mass spectrometry indicated the presence of only one cysteine residue per molecule. This was supported also by the detection of dimers of D subunits in gluten. An internal sequence of 62 amino acids preceding the cysteine was obtained, but it was not possible to identify the cysteine residue, either because it was not within the range of N-terminal sequencing of peptides obtained, or because it was present in one of the two unidentified positions.