title: Biochemical and biophysical characterization of a plant calmodulin: Role of the N- and C-lobes in calcium binding, conformational change, and target interaction.
abstract: In plants, transient elevation of intracellular Ca(2+) concentration in response to abiotic stress is responsible for glutamate decarboxylase (GAD) activation via association with calmodulin (CaM), an EF-hand protein consisting of two homologous domains (N and C). An unusual 1:2 binding mode of CaM to CaM-binding domains of GAD has long been known, however the contribution of the two CaM domains in target recognition and activation remains to be clarified. Here, we explored the coupling between physicochemical properties of Arabidopsis CaM1 (AtCaM1) and Arabidopsis GAD1 activation, focusing on each AtCaM1 lobe. We found that the four EF-loops of AtCaM1 differently contribute to the ~20 μM apparent affinity for Ca(2+) and the C-lobe shows a ~6-fold higher affinity than N-lobe (Kd(app) 5.6 μM and 32 μM for C- and N-lobes, respectively). AtCaM1 responds structurally to Ca(2+) in a manner similar to vertebrate CaM based on comparison of Ca(2+)-induced changes in hydrophobicity exposure, secondary structure, and hydrodynamic behavior. Molecular dynamics simulations of AtCaM1 apo and Ca(2+)-bound reveal that the latter state is significantly less flexible, although regions of the N-lobe remain quite flexible; this suggests the importance of N-lobe for completing the transition to the extended structure of holoprotein, consistent with data from ANS fluorescence, CD spectroscopy, and SEC analysis. Moreover, enzymatic analysis reveal that mutations in the two lobes affect GAD1 activation in similar ways and only intact AtCaM1 can fully activate GAD1. Taken together, our data provide new insights into the CaM lobes role in interactions between CaM and plant GAD.
authors.affiliation: Department of Biotechnology, University of Verona, Verona, Italy. Electronic address: alessandra.astegno@univr.it.
authors.forename: Alessandra
authors.initials: A
authors.lastname: Astegno
authors.affiliation: Department of Biotechnology, University of Verona, Verona, Italy.
authors.forename: Valentina
authors.initials: V
authors.lastname: La Verde
authors.affiliation: Department of Neurological, Biomedical and Movement Sciences, University of Verona, Verona, Italy.
authors.forename: Valerio
authors.initials: V
authors.lastname: Marino
authors.affiliation: Department of Neurological, Biomedical and Movement Sciences, University of Verona, Verona, Italy.
authors.forename: Daniele
authors.initials: D
authors.lastname: Dell'Orco
authors.affiliation: Department of Biotechnology, University of Verona, Verona, Italy.
authors.forename: Paola
authors.initials: P
authors.lastname: Dominici
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000201:enzymology; D029681:Arabidopsis Proteins / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D001703:Biophysics; D002118:Calcium / Q000378:metabolism*; D002147:Calmodulin / Q000737:chemistry* / Q000378:metabolism; D020832:EF Hand Motifs; D005968:Glutamate Decarboxylase / Q000737:chemistry* / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D017434:Protein Structure, Tertiary
chemical_list: D029681:Arabidopsis Proteins; D002147:Calmodulin; C572690:AT5G37780 protein, Arabidopsis; D005968:Glutamate Decarboxylase; D002118:Calcium
keywords: Calcium binding; Conformational change; Glutamate decarboxylase activation; Plant calmodulins; Site-directed mutagenesis