Annexin V is an abundant eukaryotic protein that binds phospholipid membranes in a Ca2؉ -dependent manner. In the present studies, site-directed mutagenesis was combined with x-ray crystallography and solution liposome binding assays to probe the functional role of a cluster of interfacial basic residues in annexin V. Four mutants were investigated: R23E, K27E, R61E, and R149E. All four mutants exhibited a significant reduction in adsorption to phospholipid membranes relative to the wild-type protein, and the R23E mutation was the most deleterious. Crystal structures of wild-type and mutant proteins were similar except for local changes in salt bridges involving basic cluster residues. The combined data indicate that Arg 23 is a major determinant for interfacial phospholipid binding and participates in an intermolecular salt bridge that is key for trimer formation on the membrane surface. Together, crystallographic and solution data provide evidence that the interfacial basic cluster is a locus where trimerization is synergistically coupled to membrane phospholipid binding.Annexins comprise a large family of homologous eukaryotic proteins with a common core region that promotes Ca 2ϩ -dependent binding to phospholipid membrane surfaces (for a review, see Refs. 1-5). This property underlies many proposed in vivo annexin functions, including membrane trafficking, cell signaling, and roles in inflammatory and coagulation processes. Annexin V is an abundant protein that binds preferentially to acidic phospholipid membranes in the presence of Ca 2ϩ . The annexin-phospholipid association is of high affinity but is reversible with the removal of Ca 2ϩ . When bound to the membrane surface, annexin V molecules assemble laterally into well organized trimers and higher order arrays of trimers (6). This lateral assembly promotes spontaneous two-dimensional crystallization of annexin V on membrane surfaces, a process that is inhibited by extreme membrane surface curvature (7,8). Annexin-coated membrane surfaces undergo changes in their properties, becoming markedly rigid as the crystallization domains increase in size (9 -11). This membrane-bound layer of annexin V, which has anticoagulant properties, is believed to play a functional role in processes associated with the protein. In placenta, reduced levels of annexin V occur with anti-phospholipid syndrome, which causes hypercoagulation and recurrent miscarriage. Rand and co-workers (12, 13) have proposed a mechanism for anti-phospholipid syndrome in which the disease process strips off the protective layer of annexin V at the maternal-fetal interface, permitting excessive thrombosis to occur.The mechanism of membrane adsorption of annexin V has generated considerable interest from many disciplines, from clinical to structural biology. The protein provides an excellent model for elucidating complex interfacial behavior by peripheral membrane proteins. The molecular structure of the 35-kDa protein has been well characterized in both soluble and membrane-bound forms by x-ray ...