Figure 3

The top of the FS domain. Besides Ca²⁺-binding site, two of three SO₄^2- groups (from crystallization buffer) identified in the structure are associated with R226 and R227 on one side of the top of the FS domain. The top of the FS domain is also rich in aromatic residues and prolines. The β7-β8 loop covers most the of FS domain on one side. When it turns at Y363, the sidechain of Y363 dips into a pocket containing aromatic residues and prolines, including P388 and F389. After the β7" short strand, four rings from H384, P385, Y390 and P392 are parallel to each other in a row. Additional aromatic residues from other loops are also found on the top of the domain, including W215, W231, P324, W326, P352, W353, Y363 and F383. All of these residues are either identical or highly conserved in the FS domains of other proteins (Figure 4). The role of these aromatic residues is unknown.