A 22-year-old man has a sudden loss of vision in the right eye. On physical examination, there is a subluxation of the right crystalline lens. On auscultation of the chest, a midsystolic click is audible. An echocardiogram shows a floppy mitral valve and dilated aortic arch. The patient's brother and his cousin are similarly affected. He is prescribed a beta-blocker. A genetic defect involving which of the following substances is most likely to be present in this patient?