The structure of the PH-domain truncated core of rat phosphoinositide-specific phospholipase C-delta 1 has been determined at 2.4 A resolution and compared to the structure previously determined in a different crystal form. The stereochemical relationship between the EF, catalytic, and C2 domains is essentially identical. The Ca2+ analogue Sm3+ binds at two sites between the jaws of the C2 domain. Sm3+ binding ejects three lysine residues which bridge the gap between the jaws and occupy the Ca2+ site in the apoenzyme, triggering a conformational change in the jaws. The distal sections of the C2 jaws move apart, opening the mouth by 9 A and creating a gap large enough to bind a phospholipid headgroup.