C-terminal residues of plant glutamate decarboxylase are required for oligomerization of a high-molecular weight complex and for activation by calcium/calmodulin

Bacterial glutamate decarboxylase (GAD) is a homohexameric enzyme of about 330 kDa. Plant GAD differs from the bacterial enzyme in having a C-terminal extension of 33 amino acids within which resides a calmodulin (CaM)-binding domain. In order to assess the role of the C-terminal extension in the formation of GAD complexes and in activation by Ca²⁺/CaM, we examined complexes formed with the purified full-length recombinant petunia GAD expressed in E. coli, and with a 9 amino acid C-terminal deletion mutant (GADΔC9). Size exclusion chromatography revealed that the full-length GAD formed complexes of about 580 kDa and 300 kDa in the absence of Ca²⁺/CaM, whereas in the presence of Ca²⁺/CaM all complexes shifted to ∼680 kDa. With deletion of 9 amino acids from the C-terminus (KKKKTNRVC₅₀₀), the ability to bind CaM in the presence of Ca²⁺, and to purify it by CaM-affinity chromatography was retained, but the formation of GAD complexes larger than 340 kDa and enzyme activation by Ca²⁺/CaM were completely abolished. Hence, responsiveness to Ca²⁺/CaM is associated with the formation of protein complexes of 680 kDa, and requires some or all of the nine C-terminal amino acid residues. We suggest that evolution of plant GAD from a bacterial ancestral enzyme involved the formation of higher molecular weight complexes required for activation by Ca²⁺/CaM.