A 28-year-old man was found to have elevated cholesterol levels of 325 mg/dL on a routine checkup. His father died of a hea attack at the age of 42, and also had markedly elevated cholesterol levels throughout his life. The man's physician placed him on lovastatin, and his cholesterol levels dropped to 170 mg/dL. The nature of the elevated cholesterol in this patient is most likely due to a mutation in which one of the following proteins?