Structural basis for simultaneous binding of two carboxy-terminal peptides of plant glutamate decarboxylase to calmodulin

Activation of glutamate decarboxylase (GAD) by calcium-bound calmodulin (CaM) is required for normal plant growth through regulation of γ-aminobutyrate and glutamate metabolism. The interaction of CaM with the C-terminal domain of GAD is believed to induce dimerization of the enzyme, an event implicated for Ca²⁺-dependent enzyme activation. Here, we present the solution structure of CaM in complex with a dimer of peptides derived from the C-terminus of Petunia hybrida GAD. The 23kDa ternary complex is pseudo-symmetrical with each domain of CaM bound to one of the two antiparallel GAD peptides, which form an X-shape with an interhelical angle of 60°. To accommodate the dimeric helical GAD target, the two domains of CaM adopt an orientation markedly different from that seen in other CaM-target complexes. Although the dimeric GAD domain is much larger than previously studied CaM-binding peptides, the two CaM domains appear closer together and make a number of interdomain contacts not observed in earlier complexes. The present structure of a single CaM molecule interacting with two target peptides provides new evidence for the conformational flexibility of CaM as well as a structural basis for the ability of CaM to activate two enzyme molecules simultaneously.