ANNEXIN-V PERTURBS OR STABILIZES PHOSPHOLIPID-MEMBRANES IN A CALCIUM-DEPENDENT MANNER

The potency of annexin V to transport Ca2+ ions across phospholipid membranes was investigated, using large unilamellar phospholipid vesicles loaded with the Ca2+ indicator fura-2. It was demonstrated that annexin V leaves the vesicle membranes intact when added in the presence of 1 mM Ca2+ However, if the vesicles were first incubated with annexin V in the absence of Ca2+, subsequent addition of Ca2+ produced a fluorescence signal due to binding of Ca2+ to fura-2. Centrifugation of the vesicle suspension immediately thereafter showed that this signal originated from the supernatant and not from the sedimented vesicles. Our results show that annexin V causes loss of vesicle integrity in the absence of Ca2+, and leakage of trapped fura-2, rather than inward Ca2+ transport. Bovine serum albumin or Ca2+ concentrations higher than 2.5 mM also caused such fura-2 leakage. Apparently, calcium-dependent binding of annexin V to the membrane prevents aspecific membrane damage caused by this protein.