Functional redundancy and formin-independent localization of tropomyosin isoforms in Saccharomyces cerevisiae

Tropomyosin is an actin binding protein which protects actin filaments from cofilin-mediated disassembly. Distinct tropomyosin isoforms have long been hypothesized to differentially sort to subcellular actin networks and impart distinct functionalities. Nevertheless, a mechanistic understanding of the interplay between Tpm isoforms and their functional contributions to actin dynamics has been lacking. In this study, we present acetylation-mimic engineered mNeonGreen-Tpm fusion proteins that exhibit complete functionality as a sole copy, surpassing limitations of existing probes and enabling real-time dynamic tracking of Tpm-actin filaments in vivo . Using these functional Tpm fusion proteins, we find that both Tpm1 and Tpm2 indiscriminately bind to actin filaments nucleated by either formin isoform-Bnr1 and Bni1 in vivo , in contrast to the long-held paradigm of Tpm-formin pairing. We also show that Tpm2 can protect and organize functional actin cables in absence of Tpm1. Overall, our work supports a concentration-dependent and formin-independent model of Tpm-actin binding and demonstrates for the first time, the functional redundancy of the paralog Tpm2 in actin cable maintenance in S. cerevisiae .