Target of rapamycin (Tor) is an evolutionarily conserved protein kinase that senses nutritional states of the cell. Tor forms two distinct complexes: Tor complex 1 (TORC1), which is sensitive to the drug rapamycin, and Tor complex 2 (TORC2), which is not. Saccharomyces cerevisiae has two TORC1 complexes: Tor1-containing TORC1 and Tor2-containing TORC1 (Tor2-TORC1). Although TORC1 and TORC2 have different functions, it remains elusive whether the two TORC1 complexes play distinct roles. In this work, Kosugi and colleagues (Kamada et al., 2024) computationally model Tor2-TORC1 and TORC2 structures to identify regions of Tor2 that are important for formation of each complex. The authors design a Tor2 mutant [Tor2(K12)] that loses the ability to form TORC1 but retains the ability to form TORC2. Here, the tor2(K12) mutant strain is more sensitive to rapamycin and caffeine and grows poorly at a high pH compared to a mutant lacking Tor1. Furthermore, the tor2(K12) strain has replicative and chronological lifespans similar to those of the wild-type strain, whereas the Tor1-deficient mutant has a longer lifespan phenotype. These observations indicate that the two TORC1 complexes have distinct functions. The methodology used paves the way to more studies addressing the roles of Tor2 and the complex evolutionary dynamics of the TOR signalling pathway. Furthermore, three-dimensional-structure-based engineering of target protein complexes can potentially be a method to address biological questions for various protein complexes.
