Functional Consequences of Mutations in the Myosin Regulatory Light Chain Associated with Hypertrophic Cardiomyopathy

Familial hypertrophic cardiomyopathy (FHC) is an autosomal dominant disease characterized by left ventricular wall thickening, myofilament disarray and abnormal echocardiography findings. Molecular genetic studies have defined FHC as a disease of the sarcomere caused by mutations in all major sarcomeric proteins, such as ┚-myosin heavy chain (44%), myosin binding protein C (35%), regulatory light chain (2%), essential light chain (1.6%), ┙-tropomyosin (2.5%), troponin T (7%), troponin I (5%), troponin C (~1%), ┙actin (1%), and titin (<1%) (Alcalai et al., 2008). Although mutations in the regulatory light chain (RLC) of myosin are rare, they are of great significance given the importance of RLC for muscle contraction and heart function. The RLC plays an essential structural and functional role by supporting the architecture of the myosin neck region and fine-tuning the kinetics of the actin-myosin interaction (Morano, 1999; Szczesna, 2003). As shown in Fig. 1, the RLC wraps around the ┙-helical neck region of the myosin head by binding to a 35 amino acid IQ motif in the myosin heavy chain (MHC) (Rayment et al., 1993). This domain of MHC is anticipated to act as a lever arm, amplifying small conformational changes that originate at the catalytic site into large movements thus allowing myosin to generate motion and force (Geeves & Holmes, 2005; Lowey et al., 1993). Furthermore, this neck region has been proposed to serve as the compliant element of the myosin cross-bridge with the RLC contributing to the stiffness of the lever arm (Howard & Spudich, 1996; Pant et al., 2009). Two functionally important domains of the RLC molecule include its Ca2+-Mg2+ binding site, comprised of the Nterminal helix-loop-helix EF-hand Ca2+ binding motif, and a highly conserved N-terminal phosphorylatable serine constituting a myosin light chain kinase (MLCK)-dependent phosphorylation site. The N-terminal divalent cation-binding site of the RLC is thought to be occupied by Mg2+ when muscles are in the relaxed state and may become partially saturated with Ca2+, depending on the length of the [Ca2+] transient (Robertson et al., 1981). The MLCK phosphorylation site of RLC is also of great structural and functional importance. Phosphorylation of this site in smooth muscle activates contraction (Hartshorne & Mrwa, 1982; Small & Sobieszek, 1977; Sobieszek, 1977). In skeletal and cardiac muscle, Ca2+-