# The TORC2-Dependent Signaling Network in the Yeast _Saccharomyces cerevisiae_
## 6 Structure, Function and Regulation of Pkc1
### Modulation of Triacylglycerglycerol Synthesis  
As described above, for synthesis of glycerophospholipids, PtdOH generated by fatty acylation of Glo3P can react with CTP to form CDP-DAG and pyrophosphate; the CDP-DAG reacts with inositol, glycerol or serine, releasing CMP and forming PtdIns, PtdGo and PtdSer. PtdSer can be converted (by decarboxylation) to PtdEth, which can, in turn, be N-trimethylated to form PtdCho. However, PtdEth and PtdCho can also be generated by an alternate route (Kennedy pathway). Ethanolamine and choline are converted to P-Eth and P-Cho by cognate ATP-dependent kinases; P-Eth and P-Cho then react with CTP to form CDP-Eth and CDP-Cho, which can react with DAG, releasing CMP and forming PtdEth and PtdCho. Thus, during active cell growth, both routes of glycerophospholipid production place a high demand on the supply of CTP and, not surprisingly therefore, the major yeast CTP synthetase isozyme (Ura7), which forms CTP by catalyzing the ATP-dependent transfer of the amide nitrogen of Gln to UTP, is subject to metabolic regulation [268].  
Moreover, in addition to its biosynthetic role in the Kennedy pathway, DAG also has a different physiological function: when fatty acyl-CoAs are in excess of the needs of the cell, they can be used to esterify DAG, forming triacylglycerglycerol for storage in lipid droplets. Because DAG has these two distinct metabolic fates, it is also not surprising that Pah1, the specific phosphatase that generates DAG from PtdOH [269], would also be subject to tight regulation to maintain lipid homeostasis because Pah1 action directly controls the relative cellular concentrations of its substrate PtdOH and its product, DAG. The relative levels of these intermediates will, in turn, affect the rates of membrane biogenesis versus lipid droplet formation.  
Indeed, the subcellular location/membrane binding [270], rate of degradation [271], and PtdOH phosphatase activity of Pah1 [272] are regulated by phosphorylation and dephosphorylation. In this regard, it has been reported, over time, that Pah1 is a target for phosphorylation by Cdk1 (Cdc28-Clb), by PKA (although, which yeast isoform, Tpk1, Tpk2, and Tpk3, was not addressed), by Cdk5 (Pho85-Pho80), by casein kinase II (Cka1-Cka2-Ckb1-Ckb2 heterotetramer in yeast) [273], and also by Pkc1 [233]. Dephosphorylation of Pah1 stimulates its catalytic activity and facilitates its association with membranes, wherein its substrate resides; dephosphorylation of Pah1 is mediated by an ER-associated heterodimeric phosphoprotein phosphatase Nem1-Spo7. Like Pah1, both Nem1 and Spo7 are purported substrates for many of the same protein kinases, including Pkc1 [233], although there is also compelling evidence that activating phosphorylation of Nem1-Spo7 is controlled by a protein kinase that is negatively regulated by TORC1 [274]. Likewise, it has been reported that the activity of Ura7 CTP synthetase is also stimulated by Pkc1-mediated phosphorylation [165,233]. Indeed, a site in Pah1, a site in Nem1, and a site in Ura7 all match well the elements of the minimal consensus defined by mutagenesis of the pseudosubstrate sequence and in comparison to the mapped sites in Bck1 (Figure 7). By contrast, three other reported sites in Ura7 and other reported sites in Spo7 and Nem1 do not. Assuming the Pkc1-mediated phosphorylation of Pah1, Nem1, and Ura7 has some collective physiological consequence in the cell, then, with respect to overall control of lipid homeostasis, Ypk1 and Pkc1 would share a related biological function, albeit at very different entry points of the glycerophospholipid metabolic network.