Cardiac leiomodin2 binds to the sides of actin filaments and regulates the ATPase activity of myosin

Abstract

Leiomodin proteins are vertebrate homologues of tropomodulin, having a role in the assembly and maintenance of muscle thin filaments. Leiomodin2 contains an N-terminal tropomodulin homolog fragment including tropomyosin-, and actin-binding sites, and a C-terminal Wiskott-Aldrich syndrome homology 2 actin-binding domain. The cardiac leiomodin2 isoform associates to the pointed end of actin filaments, where it supports the lengthening of thin filaments and competes with tropomodulin. It was recently found that cardiac leiomodin2 can localise also along the length of sarcomeric actin filaments. While the activities of leiomodin2 related to pointed end binding are relatively well described, the potential side binding activity and its functional consequences are less well understood. To better understand the biological functions of leiomodin2, in the present work we analysed the structural features and the activities of Rattus norvegicus cardiac leiomodin2 in actin dynamics by spectroscopic and high-speed sedimentation approaches. By monitoring the fluorescence parameters of leiomodin2 tryptophan residues we found that it possesses flexible, intrinsically disordered regions. Leiomodin2 accelerates the polymerisation of actin in an ionic strength dependent manner, which relies on its N-terminal regions. Importantly, we demonstrate that leiomodin2 binds to the sides of actin filaments and induces structural alterations in actin filaments. Upon its interaction with the filaments leiomodin2 decreases the actin-activated Mg2+-ATPase activity of skeletal muscle myosin. These observations suggest that through its binding to side of actin filaments and its effect on myosin activity leiomodin2 has more functions in muscle cells than it was indicated in previous studies.

In vivo elongation of thin filaments results in heart failure

“…First, excess soluble Lmod2 may bind to the sides of actin (thin) filaments and effectively limit the interaction of thin and thick filaments. This idea is based on reports that excess Lmod2 in isolated cardiomyocytes can assemble along the length of the thin filaments and interfere with proper myosin cross-bridge formation leading to a decrease in force production [11,49]. Second, our TEM data show that Lmod2-TG hearts have disorganized myofibrils, wider Z-discs and ruptured intercalated discs.…”

Role of intrinsic disorder in muscle sarcomeres

“…71 Also, endogenous Lmod was shown to localize sparsely along the length of thin filaments, 73 and the same was shown for overexpressed Lmod. 72 When F-actin was cosedimented with Lmod2, the amount of Lmod2 presented is in excess of the amount that would sediment if Lmod2 only bound to the pointed end, 71,76,77 providing further evidence that Lmod2 binds to sides of actin filaments.…”

Cardiac CapZ regulation during acute exercise

“…In this model increased contractility stimulates leiomodin to bind to actin filaments near the Z-disc and remain bound as actin treadmills through the filament. Leiomodin binding stabilizes the thin filament structure and maintains contractility in the face of an increase in demand (26,27). The ability of leiomodin to stabilize the dynamically changing myofilaments in exercising wildtype mice but not CapZ-deficient transgenic mice could explain the enhanced performance of wildtype mice over their CapZ-deficient counterparts.…”