Toward protein encapsulation by a synthetic host, we synthesized an ubiquitin-dangled ligand, a potential precursor of a nanoscale coordination cage. The key step is the addition of a C-terminal Cys76 SH group, which was introduced by Gly76Cys mutation, to a maleimide acceptor on the ligand. The C-terminal mutation and the SH addition to the ligand did not damage the ubiquitin moiety at all, neither structurally nor conformationally.