Plant-Specific Calmodulin-Binding Proteins

Abstract

Calmodulin CaM is the most prominent Ca2+ transducer in eukaryotic cells, regulating the activity of numerous proteins with diverse cellular functions. Many features of CaM and its downstream targets are similar in plants and other eukaryotes. However, plants possess a unique set of CaM-related proteins, and several unique CaM target proteins. This review discusses recent progress in identifying plant-specific CaM-binding proteins and their roles in response to biotic and abiotic stresses and development. The review also addresses aspects emerging from recent structural studies of CaM interactions with target proteins relevant to plants.

Calmodulin has the Potential to Function as a Ca2+-Dependent Adaptor Protein

“…Intracellular Ca 2+ transients and oscillations (Ca 2+ signals) are decoded by a large superfamily of calcium-binding proteins, the most important of which is calmodulin (CaM). [1][2][3] The prototypical CaM protein consists of four tandem helix-loop-helix "EF-hand" Ca 2+ -binding motifs that are divided into distinct N-and C-terminal globular lobes connected by a flexible linker. CaM proteins from all species including the single mammalian CaM and the many different plant CaM isoforms each undergo similar Ca 2+ -induced conformational changes involving a rearrangement of the position of its a-helices that opens distinct hydrophobic target protein-binding patches on the surface of each lobe; known as the "open" conformation (Fig.…”

“…1A) can also interact with another subset of proteins, to target CaM to certain cellular locations or facilitate Ca 2+ -independent regulatory events. [1][2][3] The CaM-dependent regulation of target proteins can occur through numerous different mechanisms. For example, Ca 2+ -CaM can relieve autoinhibition by binding to a short (20-25 residue) calmodulin-binding domain (CaMBD) sequence that is adjacent to or within an autoinhibitory region of the enzyme ( Fig.…”

“…30 With the abundance of poorly characterized CaM-binding proteins in plants, many of which have CaMBD's with little sequence resemblance to the better characterized motifs in animals 1 it seems likely that sequences will be identified which bind preferentially to the CaM N-lobe. Considering the incredible assortment of known CaM interaction modes and regulatory mechanisms, many of which have only been identified within the last decade, it is likely only a matter of time before CaM is proven to function as an adaptor protein in vivo.…”

Nitrogen metabolism in plants under low oxygen stress

“…Hypoxia-induced NO promotes Ca 2+ release from cellular pools thus raising free cellular Ca 2+ concentration (Garcia-Mata et al 2003). GDC is activated by acidic pH and relieved from auto-inhibition by Ca 2+ /CAM complex (Bouché et al 2004(Bouché et al , 2005. NR activity is increased by virtue of its low pH optimum (Stoimenova et al 2007).…”

“…GDC activity increases as the cytoplasmic pH declines and decreases when the pH again reaches its normal level (Carroll et al 1994). Furthermore, GDC is relieved from auto-inhibition by the Ca 2+ /Calmodulin complex (Bouché et al 2004(Bouché et al , 2005. Decarboxylation of glutamate by GDC is part of the GABA shunt that involves the reactions catalyzed by the enzymes GABA-Transaminase (GABA-T) and succinic semialdehyde dehydrogenase (SSADH).…”