Cystic fibrosis transmembrane conductance regulator and H+ permeability in regulation of Golgi pH.

This paper reviews experiments from this lab that have tested the hypothesis that pH of the Golgi (pH(G)) of cystic fibrosis (CF) airway epithelial cells is alkaline compared to normal, that this altered pH affects sialyltransferase and other Golgi enzymes controlling biochemical composition of the plasma membrane and that altered surface biochemistry increases bacterial binding. We generated a plasmid encoding a modified green fluorescence protein-sialyltransferase (GFP-ST) chimera protein that was pH-sensitive and localized to the Golgi when transfected into HeLa cells and also CF and normal or cystic fibrosis transmembrane conductance regulator- (CFTR)-corrected airway epithelial cells. Digital imaging microscopy of these Golgi-localized probes showed that there was no correlation between pH(G) (6.4-7.0) and the presence of CFTR, whether cells were in HCO(3)(-)/CO(2)-containing or in HCO(3)(-)/CO(2)-free solutions. Activation of CFTR by raising cell [cAMP] had no effect on pH(G). Thus, CFTR seemed not to be involved in controlling pH(G). Experiments on HeLa cells using an avidin-sialyltransferase chimera in combination with a pH-sensitive fluorescent biotin indicated that even in cells that do not express CFTR, Cl(-) and K(+) conductances of the Golgi and other organelle membranes were large and that pH(G) was controlled solely by the H(+) v-ATPase countered by a H(+) leak. A mathematical model was applied to these and other published data to calculate passive H(+) permeability (P(H+)) of the Golgi, endoplasmic reticulum, trans-Golgi network, recycling endosomes and secrety granules from a variety of cells. An organelle's acidity was inversely correlated to its calculated P(H+). We conclude that the CFTR plays a minor role in organelle pH regulation because other (Cl(-) and K(+)) channels are present in sufficient numbers to shunt voltages generated during H(+) pumping. Acidity of the Golgi (and perhaps other organelles) appears to be determined by the activity of H(+) pumps countered by H(+) leaks.