Close-up stereo views of the γ chain polymerization site “a” showing the interactions within the polymerization pocket for both the uncomplexed protein (A) and for the complex with GPRP (B), based on the rFbgγC30 structures. The mutation sites that affect the polymerization pocket are indicated in ball-and-stick representation. In (A), the water molecules that form hydrogen bonds with the displayed side chains are shown as pink spheres. In (B), the peptide GPRP is represented in pink. The calcium ion is shown in green, and is enlarged for emphasis.