Background: Annexin A2 is a phospholipid binding protein implicated in endo-and exocytosis. Results: Annexin A2 induces microdomain formation and inward vesicle budding in giant unilamellar vesicles. Conclusion: Specific lipid clustering and vesicle budding is triggered by annexin A2. Significance: Membrane binding by the cytosolic protein annexin A2 can induce the formation of microdomains rich in cholesterol and phosphatidylinositol 4,5-bisphosphate.

Biological membranes are organized into dynamic microdomains that serve as sites for signal transduction and membrane trafficking. The formation and expansion of these microdomains are driven by intrinsic properties of membrane lipids and integral as well as membrane-associated proteins. Annexin A2 (AnxA2) is a peripherally associated membrane protein that can support microdomain formation in a Ca(2+)-dependent manner and has been implicated in membrane transport processes. Here, we performed a quantitative analysis of the binding of AnxA2 to solid supported membranes containing the annexin binding lipids phosphatidylinositol-4,5-bisphosphate and phosphatidylserine in different compositions. We show that the binding is of high specificity and affinity with dissociation constants ranging between 22.1 and 32.2 nM. We also analyzed binding parameters of a heterotetrameric complex of AnxA2 with its S100A10 protein ligand and show that this complex has a higher affinity for the same membranes with Kd values of 12 to 16.4 nM. Interestingly, binding of the monomeric AnxA2 and the AnxA2-S100A10 complex are characterized by positive cooperativity. This cooperative binding is mediated by the conserved C-terminal annexin core domain of the protein and requires the presence of cholesterol. Together our results reveal for the first time, to our knowledge, that AnxA2 and its derivatives bind cooperatively to membranes containing cholesterol, phosphatidylserine, and/or phosphatidylinositol-4,5-bisphosphate, thus providing a mechanistic model for the lipid clustering activity of AnxA2.

Annexin A2 (AnxA2), a Ca2+-regulated phospholipid binding protein involved in membrane-cytoskeleton contacts and membrane transport, exists in two physical states, as a monomer or in a heterotetrameric complex mediated by S100A10. Formation of the AnxA2-S100A10 complex is of crucial regulatory importance because only the complex is firmly anchored in the plasma membrane, where it functions in the plasma membrane targeting/recruitment of certain ion channels and receptors. The S100A10 binding motif is located in the first 12 residues of the AnxA2 N-terminal domain, but conflicting reports exist as to the importance of N-terminal AnxA2 acetylation with regard to S100A10 binding. We show here that AnxA2 is subject to N-terminal modification when expressed heterologously in Escherichia coli. Met1 is removed and Ser2 is acetylated, yielding the same modification as the authentic mammalian protein. Bacterially expressed and N-terminally acetylated AnxA2 binds S100A10 with an affinity comparable to AnxA2 from porcine tissue and is capable of forming the AnxA2-S100A10 heterotetramer. Complex formation is competitively inhibited by acetylated but not by non-acetylated peptides covering the N-terminal AnxA2 sequence. These results demonstrate that N-terminal acetylation of AnxA2 is required for S100A10 binding and that this common eukaryotic modification is also obtained upon expression in bacteria.