Review Question - QID 213889

A 49-year-old man presents to the emergency department with a 2-day history of a painful rash. He says that he has been getting painful ulcers in his mouth for about 3 weeks but assumed that it was due to stress at his job. He became concerned when he realized that the skin on his back was starting to peel and leave painful blisters that pop with gentle pressure. The pain from these blisters and wounds have prevented him from sleeping, so he came for evaluation. On physical exam, he is found to have the rash seen in Figure A. Intact blisters are found to pop with light pressure with sloughing of nearby skin. The function of which of the following proteins is most likely disrupted in this patient?