Thin Filament Length Regulation Function of Leiomodin is Affected by its Affinity for Tropomyosin
Primary author: Garry Smith
Faculty sponsor: Alla Kostyukova
Primary college/unit: Voiland College of Engineering and Architecture
Striated muscle is composed of organized arrays of basic contractile units, sarcomeres, which must be assembled and maintained for proper muscle function. In sarcomeres, thin and thick filaments, composed primarily of actin and myosin respectively, slide against each other to create muscle contraction. For this system to perform, thin filament length must be strictly regulated. It is not fully understood how this is achieved. Leiomodin (Lmod) and tropomodulin (Tmod), homologous muscle proteins binding to actin and tropomyosin (Tpm), participate in regulating thin filament length. In cells, both are found at the slow-growing pointed end of the thin filament, and, according to an earlier proposed competition mechanism, the two have opposite effects; Tmod halts thin filament growth while Lmod allows it. The 3-dimensional structure of the Lmod/Tpm interaction site shows that Lmod binds to Tpm such that it can only occur at the pointed end. We hypothesize binding to Tpm is crucial for Lmod and Tmod competition for the pointed end. Based on the aforementioned 3-dimensional structure, we designed Lmod mutants to have decreased affinity for Tpm. We used circular dichroism (CD) spectroscopy to confirm the affinity and tested mutants in cardiomyocytes. CD experiments showed L24G mutant did not bind while S23L had decreased affinity. Cardiomyocyte experiments showed the L24G mutation resulted in shortened thin filaments and increased Tmod assembly at the pointed end, while the S23L mutation had no effect. This demonstrates that binding to Tpm is important for Lmod function and proper pointed end formation.