So you want to cage a protein?

Here is how it is done:
Elizabeth M. Vogel and Barbara Imperiali "Semisynthesis of unnatural amino acid mutants of paxillin: Protein probes for cell migration studies." Protein Science (2007) 16:550-556.

Paxillin is a 61-kDa protein involved in focal adhesion. Vogel and Imperiali use native chemical ligation to assemble this protein with a nitrobenzyl cage on a single amino acid residue. Solid phase peptide synthesis was used to make a large caged phosphopeptide peptide (30 aa from the N-ter, the blue part in the figure) that was ligated to the rest of the enzyme made via genetic methods (red part). This is the largest protein to be caged in this way. Hahn and Muir have made a caged Smad2 via a similar approach (Angew. Chem. (2004) 43:5800-5803). Muir has written an excellent review recently (Nature Methods (2006) 3:429-438) of this technique.