Analysis of a soluble calmodulin binding protein from fava bean roots: Identification of glutamate decarboxylase as a calmodulin-activated enzyme

The identity of a soluble 62-kD Ca²⁺-dependent calmodulin binding protein (CaM-BP) from fava bean seedlings was determined. Using ¹²⁵I-CaM overlay assays, a class of soluble CaM-BPs was detected in extracts of tissues comprising the axis of 1.5-week-old seedlings, excluding the root tip and emergent leaves. The size of these CaM-BPs was not uniform within all parts of the plant; the apparent molecular masses were 62 kD in roots, 60 kD in stems, and 64 kD in nodules. The root 62-kD CaM-BP was purified, and internal microsequence analysis was performed on the protein. A tryptic peptide derived from the CaM-BP consisted of a 13-residue sequence corresponding to a highly conserved region of glutamate decarboxylase (GAD), an enzyme that catalyzes the α-decarboxylation of glutamate to form the stress-related metabolite γ-aminobutyrate. Activity assays of partially purified, desalted, root GAD revealed a 50% stimulation by the addition of 100 μM Ca²⁺, a 100% stimulation by the addition of 100 μM Ca²⁺ plus 100 nM CaM, and no appreciable stimulation by CaM in the absence of added Ca²⁺. The demonstration that plant GAD is a Ca²⁺-CaM-stimulated enzyme provides a model in which stress-linked metabolism is modulated by a Ca²⁺-mediated signal transduction pathway.