Towards Understanding Plant Calcium Signaling through Calmodulin-Like Proteins: A Biochemical and Structural Perspective

Abstract

Ca2+ ions play a key role in a wide variety of environmental responses and developmental processes in plants, and several protein families with Ca2+-binding domains have evolved to meet these needs, including calmodulin (CaM) and calmodulin-like proteins (CMLs). These proteins have no catalytic activity, but rather act as sensor relays that regulate downstream targets. While CaM is well-studied, CMLs remain poorly characterized at both the structural and functional levels, even if they are the largest class of Ca2+ sensors in plants. The major structural theme in CMLs consists of EF-hands, and variations in these domains are predicted to significantly contribute to the functional versatility of CMLs. Herein, we focus on recent advances in understanding the features of CMLs from biochemical and structural points of view. The analysis of the metal binding and structural properties of CMLs can provide valuable insight into how such a vast array of CML proteins can coexist, with no apparent functional redundancy, and how these proteins contribute to cellular signaling while maintaining properties that are distinct from CaM and other Ca2+ sensors. An overview of the principal techniques used to study the biochemical properties of these interesting Ca2+ sensors is also presented.

Distinct Calcium Binding and Structural Properties of Two Centrin Isoforms from Toxoplasma gondii

“…It was noteworthy that EF-4 in both TgCEN1 and TgCEN2 was non-functional in terms of Ca 2+ binding, even if it does conform to the EF hand consensus sequence. This finding is reminiscent of the diversities between predicted and experimentally measured binding sites observed for other Ca 2+ binding proteins [25,41,54,55]. Of note, the presence of an aspartic acid residue instead of a glutamic acid residue at position 12 of EF-4 in TgCEN2, which is reported to greatly decrease the Ca 2+ -binding affinity and selectivity in many Ca 2+ binding proteins [16,17,33], could be the reason why, in TgCEN2, only EF-1 was found to be a functional Ca 2+ binding site.…”

Plant Hormone Signaling Crosstalks between Biotic and Abiotic Stress Responses

“…However, CMLs have low sequence similarities with CaMs. The targets of CaMs/CMLs include promoters, enzymes such as kinases and phosphatases, transcription factors, and ion channels [ 13 , 14 ]. The sequence divergence between CaMs and CMLs reflects their divergent properties of Ca 2+ binding and their specific targets.…”

Endolithic Algae Affect Modern Coral Carbonate Morphology and Chemistry

“…As calcium concentrations are usually strictly controlled in all types of organisms (White and Broadley, 2003;Dominguez, 2004;Brini et al, 2013) due to its toxic effects (Smith, 1995), passive diffusion of Ca 2+ ions within the algal thallus is highly unlikely. Instead, once taken up, the majority of calcium ions might have been attached to calcium-binding proteins acting as concentration buffers (La Verde et al, 2018). In contrast, calcium transport within cyanobacteria filaments is supposedly mediated by transmembrane calcium pumps (Garcia-Pichel et al, 2010).…”