The conformational dynamics of domain III in annexin V bound to negatively
charged phospholipid vesicles of 1-palmitoyl-2-oleoyl-sn-glycerophosphochol
ine and 1-palmitoyl-2-oleoyl-sn-glycerophosphoserine or incorporated into r
everse micelles of water/sodium bis(2-ethylhexyl) sulfosuccinate in isoocta
ne, used to mimic the phospholipid/water interface, was studied by steady-s
tate and time-resolved fluorescence of its single tryptophan residue (W187)
. Upon interaction with sonicated phospholipid vesicles in the presence of
calcium, or upon incorporation into reverse micelles without calcium, a pro
gressive 12-14 nm red shift of the fluorescence emission spectrum of W187 i
s observed. The indole environment becomes therefore more polar than in the
unbound protein. Three major lifetime populations describe the fluorescenc
e intensity decays of W187 in both systems. A long-lived excited-state popu
lation characterizes the membrane-bound state of the protein. The existence
of local conformers with different subnanosecond mobility is suggested by
specific association between lifetimes and correlation times both for the p
rotein in buffer and in interaction with the membrane surface. The interact
ion of the protein with the membrane surface preserves the existence of a r
apid unhindered rotational motion, which is coupled with all three lifetime
s. The longest lifetime is coupled to restricted motions in subnanosecond a
nd nanosecond time scales. The overall amplitude of rotation of the indole
ring is increased in the membrane-bound conformation of the protein. In rev
erse micelles, the local dynamics reported by W187 is also considerably inc
reased whereas the overall folding of the protein remains unaffected. The s
ame conformational change of domain III can therefore be provoked by differ
ent conditions: calcium binding at high concentration, mild acidic pH [Sopk
ova, J., Vincent, M., Takahashi, M., Lewit-Bentley, A., and Gallay, J. (199
8) Biochemistry 37, 11952-11970] and the interaction of the protein with th
e membrane surface. The high flexibility of domain III in the membrane-boun
d protein suggests that this domain may not be crucial for the interaction
of the protein with the membrane, in contrast with previous models. Our dat
a are compatible with atomic force microscopy results which suggest that do
main III of annexin V does not interact strongly with the membrane surface
[Reviakine, I., Bergma-Schutter, W., and Brisson, A. (1998) J. Struct. Biol
. 121, 356-361].