Labeling of Surface-Accessible Cysteine Residues in Engineered Virus-Like Particles

Begin with suspensions of mutant virus-like particles (VLPs) purified from infected tobacco leaves.

The mutant VLPs contain cysteine residues genetically inserted at specific positions in the viral coat protein.

Add a maleimide-conjugated fluorescent dye, and incubate the mixture in the dark.

The maleimide binds only to the cysteine residues exposed on the VLP surface, while cysteines buried within the protein structure remain inaccessible and stay unlabeled.

Add a reducing agent to block any unreacted dye and prevent non-specific labeling.

Load the samples into desalting columns packed with a porous resin, then centrifuge to purify the labeled VLPs.

The unreacted dyes and reducing agents diffuse into the porous resin beads and are retained, while the larger VLPs are excluded from the pores and elute first.

The fluorescently labeled VLPs can now be analyzed using gel electrophoresis.

Fluorescent bands on the gel show labeled mutants with surface-exposed cysteine residues.

After producing Capped T7-RNA transcripts from potato virus-X based vector plasmids carrying MRFV wild type and cys-mutated coat protein genes, use two aliquots of 10 microliters each to inoculate two N. benthamiana leaves