A 25-year-old man complains of eruptions of blisters on his scalp and inner surface of the groin and in his mouth. The blisters rupture easily and leave large crusted areas. Histologically, the lesions show separation of the stratum spinosum from the basal layer. The results of direct immunofluorescence microscopy for IgG are shown. Which of the following proteins is targeted by IgG autoantibody in the skin of this patient?