In budding yeast and metazoans, TOR proteins control cell growth. This is accomplished through two distinct activities: temporal growth regulation through upregulation of ribosomal processivity, and spatial growth regulation through modulation of cytoskeletal structure—reviewed in (Arsham and Neufeld 2006; Wullschleger et al. 2006).
The original identification of these distinct functions was facilitated by the antiproliferative drug rapamycin. Rapamycin is a cyclic macrolide isolated from the soil eubacterium Streptomyces hygroscopicus collected on the island of Rapa Nui, or Easter Island (Thomas et al. 2004). Treatment of yeast cells with rapamycin blocks cell growth immediately after cell division (Kunz et al. 1993). Mutations suppressing this effect mapped to three loci: null mutations mapped to the FKBP12 locus FKB, and missense mutations mapped to similar regions of two distinct TOR loci, Tor1 and Tor2, that were later shown to be rapamycin binding sites (Heitman et al. 1991). Rapamycin in the cytosol binds the immunophilin FPBP12. Rapamycin-FKBP12 complexes disrupt cell growth; in FKBP12 mutants, rapamycin binds TOR but has no inhibitory effect on TOR signaling (Koltin et al. 1991; Koser et al. 1993). Tor1 null mutants show only mild rapamycin hypersensitivity, indicating that TOR1 is functionally redundant with TOR2 (Cafferkey et al. 1993). Tor2 null mutants show lethality after a few rounds of cell division (Schmidt et al. 1996); cells show aberrant bud formation implying a defect in cytoskeletal organization distinct from rapamycin treatment. Tor1 Tor2 double mutants phenocopy rapamycin treatment: like rapamycin-treated cells, double-mutants divide but fail to grow (Helliwell et al. 1994, 1998). This genetic work led to the conclusion that there are two TOR activities: a rapamycin-sensitive activity promoting cell growth shared by TOR1 and TOR2, and a rapamycin-insensitive activity regulating cytoskeletal organization in cell division unique to TOR2.
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