Question:

I have made two fusion proteins in E. coli as antigens, but both of them are insoluble. Can you use a protein in SDS-PAGE gel slice as an antigen?

Resolution:

Insoluble antigens often make the best antigens since they remain at the site of injection longer than soluble proteins. Determine the protein concentration on the solubilized preparation then dialyze against at least a 20X volume of PBS. The protein will precipitate but you still know how much is there because of your protein assay results. Dispense the suspension to five tubes of approximately 150 µg each in volumes of less than 500 µl each.
For SDS-PAGE gel excisions, know roughly how much protein is in the gel slice.