**12. A working model**

As first mentioned in a 2017 article by Konarzewska *et al.*, a new model is now being hypothesized as to how Pah1p impacts apoptosis by regulating the V-ATPase *VMA3* gene, which in turn has a regulatory role over *HXK2* (**Figure 2**). This model starts with the availability of glucose that leads to a cytosolic pH, which creates a more alkaline internal environment, and will also activate protein kinase A [25]. This in turn causes the assembly of V-ATPase machinery and the formation of a functional V-ATPase pump. During this time, PA levels remain high while Pah1p's concentration is low. When PA is abundant due to lower Pah1p levels, it will follow the CDP-DAG pathway and create membrane phospholipids. Since V-ATPase assembly is being promoted, the VMA genes that encode these subunits are upregulated. Importantly, one of *VMA* genes yields Vma3p which is a key player in this model. Additionally, the lipid Phosphatidylinositol 3,5-bisphosphate can also help in the activation of the V-ATPase pump and as more V-ATPase pumps assemble and accumulate the alkaline cytosolic pH will be upheld [137]. Protein kinase A signaling can be stimulated by the newly assembled V-ATPase pumps, which leads to a positive feedback loop and increases the numbers of V-ATPase pumps being assembled. Additionally, there will be an upregulation of glycolysis which will lead to an abrupt changeover to fermentative growth. Regardless of whether the Ras/cAMP/PKA pathway is upstream or downstream of the V-ATPase pump assembly pathway [104, 134], Ras will be largely found by the cell membrane and inside the nucleus. Furthermore, the *HXK2* gene will exhibit maximal expression due to the high levels of glucose and the gene product Hxk2p will catalyze the reaction to convert glucose into Glucose-6-phosphate. Given the role of Hxk2p in apoptosis, the cell will have a greater resistance against acetic-acid induced apoptosis.

Conversely, when glucose is not abundantly present, the machinery for the V-ATPase pumps will undergo disassembly due to the less alkaline cytosolic environment. The *PAH1* gene will be upregulated and thus result in increased production of the active Pah1p which can execute its phosphatase activity on PA. This is turn will cause PA to be led along the pathway that converts it to DAG and ultimately TAG, thus minimizing the production of membrane phospholipids from PA. Furthermore, the increased amounts of Pah1p will repress most vacuolar membrane ATPase gene expression which also prevents further V-ATPase assembly and enhances its disassembly. Being that one of the *VMA* genes that Pah1p represses is *VMA3*, there will be less Vma3p available. Vma3p's regulation over *HXK2* leads to a decrease in Hxk2p. During this time, Ras will build up in the internal membranes and mitochondria which can ultimately cause dysfunction of the mitochondria and amplified ROS production [138]. This will therefore make cells more susceptible to acetic acid induced programmed cell death. Thus, Pah1p likely plays a major role in this apoptotic pathway given its regulatory role over Vma3p.

#### **Figure 2.**

*Proposed model first hypothesized in [134] for Pah1p as an apoptotic regulator via the regulation of Vma3p, which subsequently regulates Hxkp2. When glucose is available, V-ATPase pump assembly occurs due to the activation of PKA and cytosolic alkalinization as a result of glycolysis. Concurrently, Pah1p levels are decreased, which promotes the CDP-DAG pathway and PA being converted to membrane phospholipids. Vma3p, amongst other VMA genes, is upregulated due to lack of Pah1p and the increased levels of pump assembly uphold the cytosolic alkalinization. Furthermore, this leads to increased Hxk2p expression which results in the resistance of acetic acid induced apoptosis. Conversely, if glucose is not available then V-ATPase pumps undergo disassembly. Pah1p levels are high, which catalyzes the reaction of PA conversion to DAG which ultimately increases TAG levels. Furthermore, there is downregulation of VMA3, along with other VMA genes. This leads to decreased pump assembly, low levels of Hxk2p and an increase of Ras in the mitochondria and membranes. Ultimately, acetic acid induced apoptosis is promoted under these conditions [134].*

#### **13. Conclusion**

Overall, this review shows the connection between the V-ATPase pump and the lipid biosynthetic pathway PA phosphatase regulator, Pah1p. Based on the regulatory role that Pah1p has over the *VMA* genes and specifically the *VMA3* gene*,* a new model has emerged regarding how it can possibly influence acetic acid induced apoptosis through the regulation of V-ATPase genes. While more research studies will be needed to confirm this model, these studies have indicated how Vma3p can act as potential anti apoptotic factor in *Saccharomyces cerevisiae.* These findings thus

*The Interplay of Key Phospholipid Biosynthetic Enzymes and the Yeast V-ATPase Pump… DOI: http://dx.doi.org/10.5772/intechopen.97886*

highlight the importance Pah1p on vacuolar function and on cell induced apoptosis via Vma3p.
