A 65-year-old man has developed pruritus followed by blistering skin lesions over the trunk, legs, and arms over the past month. On physical examination, there are 1- to 4-cm tense bullae, particularly over flexural surfaces of the skin. A biopsy of one lesion is examined microscopically by direct immunofluorescence staining and shows a subepidermal bulla, with both IgG and C3 deposited linearly along the dermal-epidermal junction. He is treated with topical corticosteroids, and a month later the lesions are healed without scarring. Which of the following components of the skin has most likely been targeted by an autoantibody in this man?