A Ca2+/Calmodulin-Binding Peroxidase from Euphorbia Latex: Novel Aspects of Calcium−Hydrogen Peroxide Cross-Talk in the Regulation of Plant Defenses†,‡
journal contributionposted on 01.11.2005, 00:00 by Anna Mura, Rosaria Medda, Silvia Longu, Giovanni Floris, Andrea C. Rinaldi, Alessandra Padiglia
Calmodulin (CaM) is a ubiquitous Ca2+ sensor found in all eukaryotes, where it participates in the regulation of diverse calcium-dependent physiological processes. In response to fluctuations of the intracellular concentration of Ca2+, CaM binds to a set of unrelated target proteins and modulates their activity. In plants, a growing number of CaM-binding proteins have been identified that apparently do not have a counterpart in animals. Some of these plant-specific Ca2+/CaM-activated proteins are known to tune the interaction between calcium and H2O2 in orchestrating plant defenses against biotic and abiotic stresses. We previously characterized a calcium-dependent peroxidase isolated from the latex of the Mediterranean shrub Euphorbia characias (ELP) [Medda et al. (2003) Biochemistry 42, 8909−8918]. Here we report the cDNA nucleotide sequence of Euphorbia latex peroxidase, showing that the derived protein has two distinct amino acid sequences recognized as CaM-binding sites. The cDNA encoding for an E. characias CaM was also found and sequenced, and its protein product was detected in the latex. Results obtained from different CaM-binding assays and the determination of steady-state parameters showed unequivocally that ELP is a CaM-binding protein activated by the Ca2+/CaM system. To the best of our knowledge, this is the first example of a peroxidase regulated by this classic signal transduction mechanism. These findings suggest that peroxidase might be another node in the Ca2+/H2O2-mediated plant defense system, having both positive and negative effects in regulating H2O2 homeostasis.