Molecular Mechanism of Leiomodin Acting as a Leaky Cap at the Pointed End of Thin Filaments

Abstract

Molecular mechanisms of the assembly and maintenance of the sarcomere, the basic contractile unit of striated muscle, are not yet fully understood. There is no consensus on how thin filaments, actin polymers stabilized by tropomyosin (Tpm) polymers, are maintained at mature length. Proper thin filament length is essential for proper muscle contractions. Leiomodin (Lmod) and tropomodulin (Tmod), sharing a similar domain structure of actin- and Tpm-binding sites, are hypothesized to compete for binding to actin and Tpm at the pointed end of the thin filament. When Lmod is bound, actin monomers can be added to the thin filament slowly. When Tmod is bound, actin monomers can neither be added nor subtracted. A lack of structural data of Tpm binding to either Lmod or Tmod has led to debate over this mechanism. With our recently obtained 3-dimensional atomic structure of the binding interface between Lmod and Tpm and existing crystal structures of Lmod and Tmod actin-binding sites, we created a model of Lmod assembly at the pointed end of thin filaments, joining binding sites with links developed by molecular dynamics simulation. Based on the model, we propose a molecular mechanism of Lmod function as a leaky cap. According to this mechanism, actin binding site 1 (ABS1) in Lmod departs from its associated actin protomer moves away an actin monomer to bind, after which polymerization can occur. Once seven monomers have been added, a new Tpm protomer binds the newly added actin and to the previous Tpm protomer, competing with Lmod's single Tpm-binding site and displacing Lmod. ABS1 in Tmod, on the other hand, cannot move away to allow such polymerization because it is flanked on either side by Tpm-binding sites, fixing it in place.