J. Biol. Chem., Vol. 261, Issue 15, 6725-6729, 05, 1986
Fluorescence quenching of cytochrome b5 in vesicles with an asymmetric transbilayer distribution of brominated phosphatidylcholine
J Everett, A Zlotnick, J Tennyson and PW Holloway
Several fluorescence techniques have been used to estimate the depth, in
the membrane, of the endogenous tryptophans of membrane-bound proteins. We
reported recently the use of phosphatidylcholines specifically brominated
at different positions of the sn-2 acyl chain for this purpose (Markello,
T., Zlotnick, A., Everett, J., Tennyson, J., and Holloway, P. W. (1985)
Biochemistry 24, 2895-2901). The membranes made from these brominated
lipids will have the brominated lipid in both monolayers, and so the
estimated depth of the fluorophore will be relative to either the inner or
outer surface of the membrane, but will not distinguish between these two
extremes. To differentiate between these two models vesicles have now been
made with an asymmetric distribution of brominated lipid, by use of
phosphatidylcholine exchange protein. The asymmetric vesicles were isolated
by virtue of their density, and their asymmetry was established by addition
of an amphipathic fluorescent carbazole compound. With these vesicles it
was shown that the tryptophan in the membrane-binding domain of cytochrome
b5 which is quenched by bromolipid is located 0.7 nm below the outer
surface of the membrane vesicles, rather than 0.7 nm from the inner
surface.