Hypertrophic cardiomyopathy (HCM) is a common, autosomal dominant disorder primarily characterized by left ventricular hypertrophy and is the leading cause of sudden cardiac death in Youth. HCM is caused by Mutations in several sarcomeric proteins, with mutations in MYH7, encoding beta-MyHC, being the most common. While many mutations in the globular head region of the protein have been reported and studied, analysis of HCM-causing Mutations in the beta-MyHC rod domain has not yet been reported. To address this question, we performed an array of biochemical and biophysical assays to determine how the HCM-causing E1356K mutation affects the structure. stability, and function of the beta-MyHC rod. Surprisingly, the E1356K mutation appears to thermodynamically destabilize the protein. rather than alter the charge profile know to be essential for muscle filament assembly This thermodynamic instability appears to be responsible for the decreased ability of the protein to form filaments and may be responsible for the HCM phenotype seen in patients. (C) 2009 Elsevier Inc. All rights reserved.