Phosphoregulation of tropomyosin-actin interaction revealed using a genetic code expansion strategy

Tropomyosins are coiled-coil proteins that regulate the stability and / or function of actin cytoskeleton in muscle and non-muscle cells through direct binding of actin filaments. Recently, using the fission yeast, we discovered a new mechanism by which phosphorylation of serine 125 of tropomyosin (Cdc8), reduced its affinity for actin filaments thereby providing access for the actin severing protein Adf1/Cofilin to actin filaments causing instability of actin filaments. Here we use a genetic code expansion strategy to directly examine this conclusion. We produced in Escherichia coli Cdc8-tropomyosin bearing a phosphate group on Serine-125 (Cdc8 (PS125)), using an orthogonal tRNA-tRNA synthetase pair that directly incorporates phosphoserine into proteins in response to a UAG codon in the corresponding mRNA. We show using total internal reflection (TIRF) microscopy that, whereas E.coli produced Cdc8 (PS125) does not bind actin filaments, Cdc8 (PS125) incubated with lambda phosphatase binds actin filaments. This work directly demonstrates that a phosphate moiety present on serine 125 leads to decreased affinity of Cdc8-tropomyosin for actin filaments. We also extend the work to demonstrate the usefulness of the genetic code expansion approach in imaging actin cytoskeletal components.