Annexin V Interaction with Phosphatidylserine-Containing Vesicles at Low and Neutral pH

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Abstract
Annexin V belongs to a class of Ca2+-binding proteins for which different functions in the cell are discussed, e.g., involvement in exocytosis, inhibition of protein kinase C, and calcium channel activity in cartilage matrix vesicles. All these functions are related to the ability of the annexins to bind to acidic phospholipids. In this study, the interaction of annexin V with large unilamellar vesicles (LUV) prepared from phosphatidylserine (PS) at low pH was compared to that at neutral pH. Annexin V strongly binds to PS LUV at low pH, whereas at neutral pH 20-100 microM Ca2+ are required to induce binding. This is caused by the different electric charge of the protein. The binding ability of the PS LUV for annexin V is higher at low pH. Binding of annexin V induces dehydration of the vesicle surface and a decrease of the lateral diffusion within the bilayer. While this dehydration is due to vesicle contact at pH 4, at pH 7.4 it is due to surface covering by annexin V. Annexin V promotes the phospholipid intermixing between LUVs at pH 5 and below but inhibits it at pH 7.4. A substitution of up to 80% of the PS by the uncharged phosphatidylcholine does not impair the extent of phospholipid intermixing at pH 4. The high binding capacity of PS LUV, the disappearance of the inhibiting action, and a calculated increase of the annexin V hydrophobicity make it likely that annexin V is able to penetrate into the membrane at low pH. At neutral pH, annexin V molecules act as steric barriers, preventing close apposition of two vesicles. At pH 5, annexin V lowers the threshold concentration of the Ca2+-induced phospholipid intermixing. Such a promotion is well-known for annexin VII (synexin). The effect may be related to the isoelectric points of the two annexins which have been reported as 4.8 (annexin V) and 7.0 (annexin VII), respectively.

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