This work was supported by NIH R01 GM50322 to P.M. Kane. The authors thank Dr. Rajini Rao, Johns Hopkins University for providing the yeast pHluorin plasmids and for advice on ratiometric pH measurements, and Dr. Gloria A. Martinez Munoz for working out these protocols for our lab.

1. Measurement of Vacuolar pH In Vivo Using BCECF-AM
<ol>
	<li>Grow a 50 ml liquid culture of the yeast strain to be measured in the desired medium overnight. The goal is to have cells in mid-log phase (OD600 (optical density at 600 nm) measurement of approximately 0.8 for the suspension).</li>
	<li>Pellet the yeast cells by centrifugation. Resuspend the pellet in 0.6 ml of the growth medium and transfer to a microcentrifuge tube that has been weighed previously. Pellet the cells again in a microcentrifuge at ...

<ol>
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The authors have no conflicts of interest to disclose.

We have utilized these protocols to address a number of aspects of pH homeostasis. For example, we have compared cytosolic and pH responses of wild-type and V-ATPase-deficient mutant cells 4,5. We have also examined the effects of altered growth conditions, particularly extracellular pH, on vacuolar pH response to glucose 3. Importantly, the responses we observe are both consistent with other methods of quantitative pH measurement and with biochemical data describing altered activities of the ...

pH homeostasis is a dynamic and highly regulated process in all organisms 1,2. Biochemical processes are tightly regulated by pH, and intracellular environments are tuned to narrow pH ranges to allow optimal activity of the resident enzymes. However, intracellular pH homeostasis can be challenged by rapid changes in environmental pH, metabolic shifts, and certain signaling pathways. In addition, intracellular pH can itself serve as an important signal. Finally, many organelles maintain lumenal pH values that are distinct from the surrounding cytosol and essential for organelle-specific functions.
The yeast Saccharomyces cerevisiae shares a number of pH homeostasis mechanisms with higher eukaryotes 2. In the acidic organelles of the endocytic/lysosomal pathway, pH is primarily controlled by the highly conserved vacuolar proton-translocating ATPase (V-ATPase), acting in tandem with many exchangers dependent on the pH gradient. All eukaryotic cells also have proton export mechanisms. In fungi and plants, a second, distinct proton pump at the plasma membrane, Pma1, exports metabolic protons and is believed to be the major determinant of cytosolic pH and plasma membrane potential. The genetic flexibility of S. cerevisiae and its commercial importance, have made it a very interesting and important model for studying pH homeostasis 2.
In addition to being the primary drivers of organelle acidification, V-ATPases are highly regulated enzymes and our lab is interested in understanding mechanisms of V-ATPase regulation. Towards this goal, we have been using in vivo pH measurements of vacuolar and cytosolic pH: 1) to monitor responses to changing extracellular conditions, such as glucose deprivation and readdition, 2) to examine the effects of mutations that compromise V-ATPase activity, and 3) to explore the coordination of organelle and plasma membrane proton pumps 3-5. These experiments only became possible through the development of robust ratiometric pH indicators amenable to use in yeast cells. Plant et al. first showed that BCECF (2'7'-Bis-(2-carboxyethyl)-5-(and 6)-carboxyfluorescein), which has been used widely to measure cytosolic pH in mammalian cells, accumulates in the yeast vacuole instead of the cytosol 6. This difference in BCECF localization has been attributed to the many hydrolytic enzymes in the vacuole, which are likely responsible for cleavage of the acetoxy methyl ester from BCECF-AM (acetoxymethyl ester of BCECF) and vacuolar retention 6. Ali et al. 7 further developed vacuolar pH measurement using BCECF and adapted these measurements to a fluorescence plate reader format. Brett et al. introduced yeast pHluorin as a means of measuring cytosolic pH in yeast by expressing a plasmid-borne ratiometric pH-sensitive GFP 8 under control of a yeast-specific promoter 9.
The excitation spectra of both BCECF and yeast pHluorin are sensitive to pH, so they are used as ratiometric pH indicators in which the ratio of fluorescence at two excitation wavelengths, measured at a single emission wavelength, provides a measure of pH 8,10. These yeast vacuolar and cytosolic pH sensors have been used for both single-cell and population-based measurements. Single-cell measurements 6,11 are performed by fluorescence microscopy and image analysis. Vacuolar or cytosolic fluorescence at the two wavelengths is measured for each cell. The population-based measurements are performed in either a microplate reader with appropriate fluorescence capabilities or in a fluorimeter. We have generally done our measurements in a fluorimeter, because it provides easy access for addition of components such as glucose during continuous kinetic measurements. Our current lab protocols for measurement of vacuolar and cytosolic pH are listed below; both are also easily adapted to microplate assays.

<table border="1">
 <tbody>
 <tr>
 <td>Name of the reagent</td>
 <td>Company</td>
 <td>Catalogue number</td>
 <td>Comments (optional)</td>
 </tr>
 <tr>
 <td>Spectrofluorometer</td>
 <td>Horiba Jobin Yvon</td>
 <td>Model Fluoromax-4</td>
 <td>Temperature control and stirring capability are desirable.</td>
 </tr>
 <tr>
 <td>BCECF-AM</td>
 <td>Invitrogen/Molecular Probes</td>
 <td>B1150</td>
 <td>Prepare a 12 mM stock in dry DMSO, store as aliquots at -20 &deg;C </td>
 </tr>
 <tr>
 <td>monensin</td>
 <td>Sigma</td>
 <td>M5273</td>
 <td>Toxic. </td>
 </tr>
 <tr>
 <td>nigericin</td>
 <td>Sigma</td>
 <td>N7143</td>
 <td>Toxic. </td>
 </tr>
 <tr>
 <td>MES</td>
 <td>Sigma</td>
 <td>M8250</td>
 <td></td>
 </tr>
 </tbody>
</table>

measurement of vacuolar and cytosolic ph in vivo in yeast cell suspensions

Vacuolar and cytosolic pH are highly regulated in yeast cells and occupy a central role in overall pH homeostasis. We describe protocols for ratiometric measurement of pH in vivo using pH-sensitive fluorophores localized to the vacuole or cytosol. Vacuolar pH is measured using BCECF, which localizes to the vacuole in yeast when introduced into cells in its acetoxymethyl ester form. Cytosolic pH is measured with a pH-sensitive GFP expressed under control of a yeast promoter, yeast pHluorin. Methods for measurement of fluorescence ratios in yeast cell suspensions in a fluorimeter are described. Through these protocols, single time point measurements of pH under different conditions or in different yeast mutants have been compared and changes in pH over time have been monitored. These methods have also been adapted to a fluorescence plate reader format for high-throughput experiments. Advantages of ratiometric pH measurements over other approaches currently in use, potential experimental problems and solutions, and prospects for future use of these techniques are also described.

Figure 1 presents vacuolar pH data obtained on wild-type yeast cells grown in rich medium (yeast extract-peptone-dextran; YEPD) buffered to pH 5 with 50 mM MES. We often grow the cells in buffered medium because the pH of the medium can change quite dramatically during overnight growth, particularly for minimal medium, and we have found that the pH of the growth medium can affect vacuolar pH responses 3. However, it is also acceptable for many experiments to grow the cells in unbuffered...

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Measurement of Vacuolar and Cytosolic pH In Vivo in Yeast Cell Suspensions

Vacuolar and cytosolic pH can be measured in live yeast (S. cerevisiae) cells using ratiometric fluorescent dyes localized to specific cellular compartments. We describe procedures for measuring vacuolar pH with BCECF-AM, which localizes to the vacuole in yeast, and cytosolic pH with a cytosolic ratiometric pH-sensitive GFP (yeast pHluorin).

Measurement of Vacuolar and Cytosolic pH In Vivo in Yeast Cell Suspensions

Vacuolar and  cytosolic pH are highly regulated in yeast cells and occupy a central role in  overall pH homeostasis. We describe  protocols for ...

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19.7K Views.  SUNY Upstate Medical University. Vacuolar and cytosolic pH can be measured in live yeast (S. cerevisiae) cells using ratiometric fluorescent dyes localized to specific cellular compartments. We describe procedures for measuring vacuolar pH with BCECF-AM, which localizes to the vacuole in yeast, and cytosolic pH with a cytosolic ratiometric pH-sensitive GFP (yeast pHluorin).