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?

Pemphigus vulgaris is an autoimmune disease caused by autoantibodies to a keratinocyte antigen. The characteristic lesion is a large, easily ruptured blister that leaves extensive denuded or crusted areas. Supra-basal dyshesion results in a blister that has an intact basal layer as a floor and the remaining epidermis as a roof. The blister contains a moderate number of lymphocytes, macrophages, eosinophils, and neutrophils. Distinctive, rounded keratinocytes (termed acantholytic cells) are shed into the vesicle during the process of dyshesion. Circulating IgG antibodies in patients with pemphigus vulgaris react with an epidermal surface antigen called desmoglein-3, a desmosomal protein. Antigen-antibody union results in dyshesion, which is augmented by the release of plasminogen activator and, hence, the activation of plasmin. This proteolytic enzyme acts on the intercellular substance and may be the dominant factor in dyshesion. None of the other choices are related to the pathogenesis of pemphigus vulgaris.Diagnosis: Pemphigus vulgaris