Annexins V and XII insert into bilayers at mildly acidic pH and form ion channels

The functional hallmark of annexins is the ability to bind to the surface of phospholipid membranes in a reversible, Ca2+-dependent manner. We now report that human annexin V and hydra annexin XII reversibly bound to phospholipid vesicles in the absence of Ca2+ at low pH; half-maximal vesicle association occurred at pH 5.3 and 5.8, respectively. The following biochemical data support the hypothesis that these annexins insert into bilayers at mildly acidic pH. First, a photoactivatable reagent (3-trifluoromethyl)-3-(m-[I-125]iodophenyl)diazirine) which selectively labels proteins exposed to the hydrophobic domain of bilayers reacted with these annexins at pH 5.0 and below but not at neutral pH. Second, in a Triton X-114 partitioning assay, annexins V and XII act as integral membrane proteins at low pH and as hydrophilic proteins at neutral pH; in the presence of phospholipids half-maximal partitioning into detergent occurred at pH approximate to 5.0. Finally, annexin V or XII formed single channels in phospholipid bilayers at low pH but not at neutral pH, A model is discussed in which the concentrations of H+ and Ca2+ regulate the reversible conversion of three forms of annexins-soluble, peripheral membrane, and transmembrane.