# The TORC2-Dependent Signaling Network in the Yeast _Saccharomyces cerevisiae_
## 8 Cross-Talk between Ypk1- and Pkc1-Dependent Signaling  
One early indication that TORC2 action involves the integration of different cellular signaling pathways was the isolation of temperature-sensitive _tor2_ alleles that fell into distinct phenotypic classes and the isolation of corresponding multicopy suppressors that could rescue one class, but not another [290]. Among the suppressors of a so-called Class A _tor2_ mutant were genes encoding components of the Pkc1 pathway, including the Rho1 GEF Rom2 and Pkc1 itself, whereas genes encoding enzymes of phosphoinositide metabolism (e.g., the PtdIns4P 5-kinase Mss4) were among the suppressors of a so-called Class B _tor2_ mutant [290]. These findings suggested that the functions of Tor2, and thus TORC2, involve at least two interrelated signaling pathways. The first specific indication that Ypk1-mediated processes and Pkc1-mediated processes are intertwined also came from genetic observations. Among the dosage suppressors of _ypk1 ts_\(ypk2\Delta\) cells that we isolated was _EXG1_ (the gene for a major exoglucanase) and among the extragenic suppressors created by transposon insertion were loss-of-function mutations in _KEX2_ (the gene for an endoprotease in the _trans_-Golgi compartment necessary for the processing and proper trafficking of manyoproteins and zymogens, including Exg1), suggesting that damage to the cell wall might bypass the need for full Ypk1 function by upregulating Pkc1 function and/or the cell wall integrity pathway [46]. Indeed, we found that the viability of \(ypk1^{ts}\)\(ypk2\Delta\) cells could also be maintained by _PKC1_ overexpression or by an activated allele (\(BCKI\)-20) of the downstream protein kinase and, conversely, that a \(slt2\Delta/npk1\Delta\) mutation was lethal in \(ypk1^{ts}\)\(ypk2\Delta\) cells [46], suggest that the processes Ypk1 controls to maintain PM homeostasis work in parallel with the Pkc1-controlled pathway for maintenance of cell wall integrity. Likewise, it was shown independently by others that, like Pkc1-deficient cells, Ypk-deficient cells have actin defects and reduced ability to activate Slt2/Mpk1 and, conversely, that overexpression of the Rho1 GEF Tus1, of Rho1 itself, or upregulation of the _PKC1_-controlled MAPK pathway suppressed the growth and actin defects of Ypk-deficient cells [214]. Similarly, it has been found [34] that the TORC2-Ypk1-regulated protein kinase Fpk1 is needed to control polarization of endocytic actin patches, in agreement with other recent results [26], and, furthermore, that Ypk1 influences Pkc1 activity through effects on the PM localization of the Rho1 GEF Rom2, which depends on sphingolipid synthesis and Fpk1 [34], both of which are under TORC2-Ypk1 control, reinforcing the important links between TORC2-Ypk1 signaling and Pkc1, especially at the level of effects on Pkc1 mediated via Rho1. At the mechanistic level, other recent evidence is also compatible with coupling between Ypk1 and Pkc1 signaling occurring at the level of the supply and/or localization of Rho1-GTP. First, among the suppressors of the temperature-sensitivity of both \(lst8\Delta\) and \(tor2\)-\(21^{ts}\) cells were loss-of-function mutations in the gene for the Rho1 GAP Sac7, suggesting that increasing the steady-state supply of Rho1-GTP may, in turn, upregulate Pkc1 function, which is then responsible for the rescue [291]. Second, the PM lipid most responsible for PM recruitment of Rho1 under normal conditions is PtdIns4,5P2 [221]; however, it has been reported that, under conditions of membrane stress (hypotonic shock and myriocin treatment), PM PtdIns4,5P2 is acutely degraded, but that Rho1 is nevertheless retained at the PM because activation of TORC2-Ypk1 signaling somehow causes an alternative acidic glycerophospholipid--namely, PtdSer--to be accumulated in the inner leaflet of the PM [292]. It is further claimed [292] that the observed increase in PtdSer somehow arises from preventing Fpk1-mediated stimulation of the Lem3-dependent flippases Dnf1 and Dnf2 [52,284].