A protein produced by an expression system that includes a nucleic acid sequence that encodes an MHC class I heavy chain. The MHC class I heavy chain comprises an alpha-1 helix and an alpha-2 helix. The nucleic acid sequence has a disulfide bridge formed between the alpha-1 helix and the alpha-2 helix in the MHC class I heavy chain. Amino acid 139 is substituted by a cysteine so as to provide Cys-139. Amino acid 84 is substituted by the cysteine so as to provide Cys-84 or amino acid 85 is substituted by the cysteine so as to provide Cys-85. The disulfide bridge is formed between the alpha-1 helix and the alpha-2 helix in the MHC class I heavy chain between Cys-139 and Cys-84 or between Cys-139 and Cys-85. The protein comprises an anchor element selected from a natural biotinylation sequence, a polyhistidine sequence, or a polyarginine sequence.
