Rief et al. have modified GFP at 5 different positions so that they could use an atomic force microscope to pull the protein apart. They found that the force required to unfold GFP depends on the direction of the pull (see below).

GFP's directional deformation response. The width of the connecting arrows represents the average fracture force in that particular direction and the color encodes for the protein rigidity in each of the 5 directions (PNAS 2006).