The presence of polyP in tetramers of TRPM8 was detected by its metachromatic reaction to the cationic dye, o-toluidine blue. PolyP of >5 residues causes a shift in the absorption on maximum of o-toluidine blue toward shorter wavelengths,
i.e., from 630 nm (blue) to 530 nm (violet-red) [38]. PolyP stains a distinctive reddish-purple color on PAGE gels (Fig. 4A, lane 2). The identity of polyP was confirmed by its complete degradation when TRPM8 was incubated with 2 μg/ml scPPX1 (Wurst et al., 1995) for 3 h at 37°C before loading on the gel (Fig. 4A, lane 3). The presence of TRPM8 in lanes 2 and 3 of the gel was confirmed by re-staining the gel with Coomassie blue (lanes 4 and 5). The protein and polyP detected on the native gels migrate at an apparent molecular weight of 490–500 kDa, which corresponds to the molecular weight of TRPM8 in the tetrameric
form. The association of polyP with the TRPM8 protein was confirmed
after each protein purification procedure. A total of 12 native PAGE experiments
were performed for detection of polyP.