Figure 3.

The crystallographic RGS17 dimer coordinates four Ca²⁺ ions. RGS17 chains are colored as in Fig. 1. Ca²⁺ ions are shown as black spheres, waters are shown as red spheres, and the distance between the Ca²⁺ ion and the coordinating atoms are shown in dashed yellow lines. All coordination distances are between 2.3 and 2.6 Å. A, the backbone carbonyl oxygen of Tyr-106 and side chain of Glu-109 in the α3–α4 loop coordinate one Ca²⁺ ion in Chain X. This is in close proximity to Ser-150, the GAP residue in the RZ subfamily. B, as observed in A, RGS17 Chain A also coordinates Ca²⁺ via Tyr-106 and Glu-109. A second Ca²⁺ is bound by the backbone carbonyl of Ile-143 and side chain of Glu-148 in the α5–α6 loop. C, the carbonyl oxygen of Gln-124, located in the α4–α5 loop, coordinates a Ca²⁺ ion in Chain X.