# The TORC2-Dependent Signaling Network in the Yeast _Saccharomyces cerevisiae_
## 3 TORC2 Effects  
Yeast TORC2 is localized at the PM [27; 29; 35; 36] and responds to certain perturbations and stresses by directly phosphorylating two distinct types of protein kinases: Ypk1 (and its paralog Ypk2/Ykr2) [37] (Figure 1); and, Pkc1 [38] (Figure 2). TORC2 phosphorylates Ypk1 at multiple sites at its C-terminal end, thereby stimulating its activity [22; 24; 39]. Ypk1 is a member of the sub-family of eukaryotic protein kinases first defined by the related protein kinases cyclic-3'5'-AMP-dependent protein kinase (PKA), cyclic-3'5'-GMP-dependent protein kinase (PKG), and the conventional Ca\({}^{2+}\)-, DAG-, and PtdSer-activated protein kinases (PKC) (AGC-family of protein kinases) [40; 41]. Several criteria indicate that the human ortholog of Ypk1 is SGK1. The catalytic domain of Ypk1 shares a greater degree of amino acid sequence identity (57%) with that of SGK1 than any other human protein kinase [42; 43; 44]. In addition, the optimal site for phosphorylation of substrates (phospho-acceptor site specificity) of Ypk1 and SGK1 are virtually identical [45]. Moreover, expression of SGK1 (and no other, even closely related, mammalian protein kinase) rescues the inviability of a _ypk1_\(\Delta\)_ypk2_\(\Delta\) double mutant [45]. Lastly, as a prerequisite for its TORC2 stimulation, basal Ypk1 activity requires phosphorylation of its activation loop (T-loop) by the PM-localized protein kinases Pkh1 and Pkh2 [46; 47]. Pkh1 and Pkh2 are the yeast sequence homologs of the mammalian, PM-associated, 3-phosphoinositide-dependent protein kinase 1 (PDK1) and expression of PDK1 rescues the inviability of a _pkh1_\(\Delta\)_pkh2_\(\Delta\) double mutant [45]. As for Ypk1, full SGK1 activity requires its phosphorylation by both PDK1 [48] and mTORC2 [49]. For SGK1, there is some evidence that modification at one of its C-terminal mTOR2 sites, designated its \"hydrophobic motif\" (FLGFSYAPP) obligatorily precedes PDK1-mediated phosphorylation of its activation loop [53], rather than being independent events as we have observed for Ypk1 in yeast [22; 39; 47]. The crystal structure of PDK1 [54] suggested that this requisite order might arise from the fact that the N-terminal lobe of the PDK1 kinase domain contains a hydrophobic groove (\"PIF pocket\") lined with nearby basic (Lys76 and Arg131) and polar (Thr148 and Gln150) side chains capable of coordinating a negatively-charged phosphate group. Hence, it was proposed that, once phosphorylated, the hydrophobic motif of an AGC kinase docks in the PIF pocket, thereby recruiting it to PDK1. Such an intermolecular interaction would then allow PDK1 to  
Figure 1: Schematic depiction of the primary structure of Ypk1. Catalytic domain (black) and N- (light blue) and C-terminal (dark blue) regulatory elements are indicated. Shading reflects percent sequence identity between Ypk1 (680 residues) and the corresponding segment in its paralog Ypk2 (677 residues): 1–98, 22% (faint blue); 99–341, 62% (medium blue); 342–602, 90% (black); and, 603–680, 73% (dark blue). Abbreviations: LX, low-complexity sequences predicted by UnitProt [50] and/or SMART [51] databases; AL, activation loop Thr (T504), phosphorylated by Pkh1 and, less efficiently, by Pkh2 [46]; TM, turn motif Ser (S644), phosphorylated by TORC2; and, HM, hydrophobic motif Thr (T662), phosphorylated by TORC2. Two N-terminal residues phosphorylated by Fpk1 and, less efficiently, by Fpk2 [52] are also shown. Figure 2: Schematic depiction of the primary structure of Pkc1. Catalytic domain (black) and N- (white) and C-terminal (dark blue) regulatory elements are indicated.