Figure 7(A) Effects of mutation of G187 on the property of the GRAM domain of GRAMD1b (GRAM1b) to sense transient expansion of the accessible pool of PM cholesterol. Indicated EGFP-tagged mutant versions of GRAM1b were expressed in GRAMD1 TKO HeLa cells and imaged under confocal microscopy with or without SMase treatment (100 mU/ml for 1 hour at 37°C) or with SMase treatment followed by 5 min treatment with Methyl-β-cyclodextrin (MCD) (5 mM) to assess their PM recruitment. The mean values (n = 20 cells from two independent experiments for untreated and SMase-treated conditions; n = 10 cells from one experiment for SMase & MCD-treated conditions) of the ratio of PM signals to the cytosolic signals, as assessed by line scan analysis, are presented for each condition as a heatmap. Amino acids are ranked according to Goldman, Engelman and Steitz (GES) hydrophobicity scale with the most hydrophobic amino acid on the top.(B) Liposome tethering by purified GRAMD1b proteins. Left: Representative time course of liposome tethering, as assessed by turbidity of mixtures containing both LPM and LER together with either wild-type GRAMD1b proteins (WT), mutant GRAMD1b G187L proteins (G187L), or buffer alone as indicated. Experiments and data analysis were performed as in Fig 4. Right: Values of turbidity corresponding to the end of the experiment as indicated by the arrow (mean ± SEM, n = 3 independent experiments; Dunnett's multiple comparisons test, *P = 0.0143).(C) DHE transfer between liposomes by purified GRAMD1b proteins. Left: Representative time course of DHE transfer from LPM to LER mediated by either wild-type GRAMD1b proteins (WT), mutant GRAMD1b G187L proteins (G187L), or buffer alone as indicated. Methyl-β-cyclodextrin (MCD) at 1 mM was used to determine DHE equilibration. Experiments and data analysis were performed as in Fig 4. Right: Values of DHE transfer rate of wild-type GRAMD1b (WT) and mutant GRAMD1b (G187L), as estimated by the FRET-based lipid transfer assay (mean ± SEM, n = 3 independent experiments; two-tailed unpaired Student's t-test, **P < 0.0014).(D) Confocal images of live GRAMD1 TKO HeLa cells that stably expressed either EGFP control or EGFP-GRAMD1b constructs as indicated. Cells were stained with recombinant mCherry-D4H proteins (10 mg/ml) (accessible PM cholesterol biosensor) for 15 min at room temperature before imaging. Insets show at higher magnification the regions indicated by white dashed boxes. Note that expression of EGFP-GRAMD1b-WT, but not EGFP-GRAMD1b-R189W, reduced binding of mCherry-D4H to the PM, in GRAMD1 TKO HeLa cells. Further reduction in mCherry-D4H binding to the PM was observed in GRAMD1 TKO HeLa cells that stably expressed EGFP-GRAMD1b-G187L compared to GRAMD1 TKO HeLa cells that stably expressed EGFP-GRAMD1b-WT. Scale bars, 10 µm. (E) Values of mCherry-D4H signals at the PM after background subtraction, as assessed by confocal microscopy and line scan analysis of GRAMD1 TKO HeLa cells that stably expressed either EGFP or indicated EGFP-GRAMD1b constructs as shown in (D) [mean ± SEM, n = 30 cells for all the conditions; data are pooled from three independent experiments; Dunnett's multiple comparisons test, **P < 0.0001].