A 52-year-old male presents with progressive muscle weakness and difficulty swallowing. His history reveals frequent episodes of pneumonia due to aspiration. Laboratory tests show elevated serum creatine kinase (CK), and a muscle biopsy reveals misfolded proteins with abnormal disulfide bond formation. Genetic testing confirms a mutation affecting cysteine residues in a structural protein. Which of the following proteins is most likely affected by the disrupted disulfide bond formation?
