Structural Basis for Simultaneous Binding of Two Carboxy-terminal Peptides of Plant Glutamate

Activation of glutamate decarboxylase (GAD) by calcium-bound calmodulin (CaM) is required for normal plant growth through regulation of g-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 Ca2þ-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 23 kDa 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 608. To accommodate the dimeric helical GAD target, the two domains of CaM adopt an orientation markedly different from that seen in other CaM–target com-plexes. 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 inter-acting with two target peptides provides new evidence for the confor-mational flexibility of CaM as well as a structural basis for the ability of CaM to activate two enzyme molecules simultaneously