A 5-year-old male presents with a history of failure to thrive, chronic diarrhea, and easy bruising. Despite adequate caloric intake, he has experienced poor weight gain since infancy. He also has chronic, greasy stools and a history of easy bruising. His mother reports that he has delayed motor development. Physical examination reveals an underweight child with delayed motor milestones and mild ataxia. Laboratory investigations demonstrate markedly low levels of cholesterol and triglycerides. The diagnosis of abetalipoproteinemia is suspected. The pathogenesis of abetalipoproteinemia involves the critical role of which of the following proteins?
