## 5 pH in Cellular Organelles
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Sensor** & **Tag** & **pH** & **Organism/Cell Type** & **Instrument\({}^{\text{a}}\)** & **Reference** \\ \hline Large dense-core vesicles & NPY-ClopHensor & Neuropeptide Y, N-terminal signal & 5.2, 5.6 & PC12, WSS-1 cells & Micro & [45] \\ Early endosomes & Cellubrevin-r-pHluorin & Cellubrevin & 5.9 & HEK cells & Micro & [59] \\  & & Transactivator & & & \\ Endosomes & Tat-E\({}^{1}\)GFP & protein of human immunodeficiency & 6.8 early/ & & \\  & & immunodeficiency & 5.8–6.5 & HeLa cells & Micro & [50] \\  & & virus-1 & & & \\ Multivesicular bodies & PRpHluorin–AtVSR2 & Vacuolar-sorting & 6.2 & _A. thaliana_ & Micro & [4] \\ Vacuoles & aleurain–PRpHluorin & Aleurain & 5.2 & _A. thaliana_ & Micro & [4] \\ Vacuoles & RaVC & 6.2–6.5 & _A. niger_ & Micro & [33] \\ \hline \end{tabular}  
* Micro, FLIM, spec, flow stand for microscopy, fluorescence life time microscopy, spectrophotometry and flow cytometry; b Only pH changes are presented in the paper.  
\end{table} 
Table 6: _Cont._ \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sensor** & **Tag** & **pH** & **Organism/Cell Type** & **Instrument\({}^{\text{a}}\)** & **Reference** \\ \hline \multicolumn{6}{c}{Peroxisomes} \\ \hline pHluorin-SKL & SKL at C-terminus & 6.9–7.1 & \begin{tabular}{c} CHO cells, human \\ foreskin fibroblasts \\ \end{tabular} & Micro & [70] \\ pHluorin-SKL & SKL at C-terminus & 8.0 & _S. cerevisiae_ BY4743 & Flow & [55] \\ PRpHluorin-SRL & SRL at C-terminus & 8.4 & _A. thaliana_ & Micro & [4] \\ \hline \multicolumn{6}{c}{Other specialized compartments} \\ \hline Cell cortex & pHMA & \begin{tabular}{c} moesin 140 aa C-terminus \\ actin binding domain \\ \end{tabular} & 7.0–7.4 & _Drosophila_ S2 cells & Micro & [71] \\ Apoplasts & Apo-pHusion & Chitinase & b & _A. thaliana_ & Micro & [47] \\ Plastid stroma/ RecA–chloroplasts & RReAlworn & Rubisco activase & 7.2 & _A. thaliana_ & Micro & [4] \\ \hline \multicolumn{6}{c}{Mitochondria} \\ \hline mtAlpH & COX-IV (1-12 aa) & 8.1 & HeLa cells & Micro & [20] \\  & MitoSprHer & COX-VIII (1-25 aa) & 7.6 & HeLa cells & Micro & [39] \\  & ECFPmito, & COX-IV (1-12 aa) & 8.0/7.9 & \
\begin{tabular}{c} HeLa/ \\ cardiopulmonaryocytes \\ \end{tabular} & Micro & [12] \\  & mito-pHluorin & COX-IV (1-12 aa)-RSGI\({}_{\text{ linker}}\) & 7.7 & CV-1 cells & Micro & [72] \\  & COX8-GFP- & COX-VIII (1-25 aa) & \(>\)7.5 &
\begin{tabular}{c} CHO-K1, \\ LLC-PK1 \\ \end{tabular} & Micro & [11] \\  & COX8-pHRed & COX-VIII (1-25 aa) & 8.0 & Neuro 2A cells & Micro & [40] \\  & COX4-pHluorin & COX-IV (1-25 aa) & 7.7 & _S. cerevisiae_ BY4743 & Flow & [55] \\  & Mito-PpHluorin & \(\beta\)-subunit F1-ATPase & 8.1 & _A. thaliana_ & Micro & [4] \\ \hline \end{tabular}  
* Micro, FLIM, spec, flow stand for microscopy, fluorescence life time microscopy, spectrophotometry and flow cytometry; b Only pH changes are presented in the paper.  
\end{table} 
Table 7: Select examples of pH-sensitive FPs targeted to peroxis, mitochondria, and other specialized compartments. Peroxisomes [73], organelles that derived from endoplasmic reticulum, are involved in many different cellular functions, such as breakdown of fatty acids, polyamines and D-amino acids. r-pHluorin is directed to peroxisomes with peroxisomal targeting SKL/SRL peptide, and is used to measure peroxisomal pH, which range from 6.9 to 8.4 [4,55,70] (Table 7). A chitinase and an N-terminus of Rubisco activase (recA) protein fused to pHusion and PRpHluorin directed chimeric proteins to apoplasts and plastid stroma of _Arabidopsis thaliana_, respectively [4,47]. The pH of the cell cortex of _Drosophila_ S2 cells was analyzed using pHMA sensor--pHluorin tagged to moesin [71]. Recent studies on mitochondrial pH present a good demonstration of the importance of the selection of a pH sensor that matches the pH of the subcellular compartment [74] (Table 7). The resting mitochondrial matrix pH (pH\({}_{\text{M}}\)) is markedly alkaline from 7.6 to 8.1 [4,11,12,20,40,41,55,72].