  In a previous work, we have shown that a spatially localized transmembrane pH
gradient, produced by acid micro-injection near the external side of
cardiolipin-containing giant unilamellar vesicles, leads to the formation of
tubules that retract after the dissipation of this gradient. These tubules have
morphologies similar to mitochondrial cristae. The tubulation effect is due to
direct phospholipid packing modification in the outer leaflet that is promoted
by protonation of cardiolipin headgroups. Here we compare the case of
cardiolipin-containing giant unilamellar vesicles with that of
phosphatidylglycerol-containing giant unilamellar vesicles. Local acidification
also promotes formation of tubules in the latter. However, compared to
cardiolipin-containing giant unilamellar vesicles the tubules are longer,
exhibit a visible pearling and have a much longer lifetime after acid
micro-injection is stopped. We attribute these differences to an additional
mechanism that increases monolayer surface imbalance, namely inward PG
flip-flop promoted by the local transmembrane pH-gradient. Simulations using a
fully non-linear membrane model as well as geometrical calculations are in
agreement with this hypothesis. Interestingly, among yeast mutants deficient in
cardiolipin biosynthesis, only the crd1-null mutant, which accumulates
phosphatidylglycerol, displays significant mitochondrial activity. Our work
provides a possible explanation of such a property and further emphasizes the
salient role of specific lipids in mitochondrial function.
