PLANT PHYSIOLOGY , Vol 112, Issue 2 759-766, Copyright © 1996 by American Society of Plant Biologists
Mechanism of Proton Permeation through Chloroplast Lipid Membranes
B. Fuks and F. Homble
Laboratoire de Physiologie Vegetale, CP 206/2, Universite Libre de Bruxelles, B-1050 Brussels, Belgium
Electrical measurements were carried out to investigate the contribution of
chloroplast lipids to the passive proton permeability of both the thylakoid
and inner-envelope membranes. Permeability coefficient and conductance to
protons were measured for solvent-free bilayers made from
monogalactosyldiglyceride:digalactosyldiglycerid:sulfoquinovosyldiglyceride
:phosphatidylglycer ol (2:1:0.5:0.5, w/w) in the presence of a pH gradient
of 7.4/8.1. The permeability coefficient for protons in glycolipids was 5.5
[plus or minus] 1.1 x 10-4 cm s-1 (n = 14). To determine whether this high
H+ permeability could be explained by the presence of lipid contaminants
such as weak acids, we investigated the effects of (a) bovine serum
albumin, which can remove some amphiphilic molecules such as free fatty
acids, (b) 6-ketocholestanol, which increases the membrane dipole
potential, (c) oleic acid, and (d) chlorodecane, which increases the
dielectric constant of the lipid bilayer. Our results show that free fatty
acids are inefficient protonophores, as compared with
carbonylcyanide-m-chlorphenylhydrazone, and that the hypothesis of a weak
acid mechanism is not valid with glycolipid bilayers. In the presence of
deuterium oxide the H+ conductance was reduced significantly, indicating
that proton transport through the glycolipid matrix could occur directly by
a hydrogen bond process. The passive transport of H+ through the glycolipid
matrix is discussed with regard to the activity of the thylakoid ATP
synthase and the inner-envelope H+-ATPase.
