Grischa Chandy, Michael.Grabe,
Hsiao-Ping Moore and Terry Machen (2001). Proton leak and CFTR in
regulation of Golgi pH in airway epithelial cells.
Physiol. 281, C908-C921.
addressing whether cystic fibrosis transmembrane conductance regulator (CFTR)
plays a role in regulating organelle pH has remained inconclusive. We engineered
a pH-sensitive excitation ratiometric green fluorescent protein (pHERP) and
targeted it to the Golgi with sialyltransferase (ST). As determined by
ratiometric imaging of cells expressing ST-pHERP, Golgi pH (pH(G)) of HeLa cells
was 6.4, while pH(G) of mutant (DeltaF508) and wild-type CFTR-expressing (WT-CFTR)
respiratory epithelia were 6.7-7.0. Comparison of genetically matched DeltaF508
and WT-CFTR cells showed that the absence of CFTR statistically increased Golgi
acidity by 0.2 pH units, though this small difference was unlikely to be
physiologically important. Golgi pH was maintained by a H(+) vacuolar (V)-ATPase
countered by a H(+) leak, which was unaffected by CFTR. To estimate Golgi proton
permeability (P(H(+))), we modeled transient changes in pH(G) induced by
inhibiting the V-ATPase and by acidifying the cytosol. This analysis required
knowing Golgi buffer capacity, which was pH dependent. Our in vivo estimate is
that Golgi P(H(+)) = 7.5 x 10(-4) cm/s when pH(G) = 6.5, and surprisingly, P(H(+))
decreased as pH(G) decreased.