Crystal structure of human annexin I at 2.5 Å resolution

Abstract

cDNA coding for N‐terminally truncated human annexin I, a member of the family of Ca 2+‐dependent phospholipid binding proteins, has been cloned and expressed in Escherichia coli. The expressed protein is biologically active, and has been purified and crystallized in space group P2 1 2 1 2 1 with cell dimensions a = 139.36 Å, b = 67.50 Å, and c = 42.11 Å. The crystal structure has been determined by molecular replacement at 3.0 Å resolution using the annexin V core structure as the search model. The average backbone deviation between these two structures is 2.34 Å. The structure has been refined to an R‐factor of 17.7% at 2.5 Å resolution. Six calcium sites have been identified in the annexin I structure. Each is located in the loop region of the helix–loop–helix motif. Two of the six calcium sites in annexin I are not occupied in the annexin V structure. The superpositions of the corresponding loop regions in the four domains show that the calcium binding loops in annexin I can be divided into two classes: type II and type III. Both classes are different from the well‐known EF‐hand motif (type I).