Plant calmodulins and calmodulin-related proteins: multifaceted relays to decode calcium signals

Abstract

The calmodulin (CaM) family is a major class of calcium sensor proteins which collectively play a crucial role in cellular signaling cascades through the regulation of numerous target proteins. Although CaM is one of the most conserved proteins in all eukaryotes, several features of CaM and its downstream effector proteins are unique to plants. The continuously growing repertoire of CaM-binding proteins includes several plant-specific proteins. Plants also possess a particular set of CaM isoforms and CaM-like proteins (CMLs) whose functions have just begun to be elucidated. This review summarizes recent insights that help to understand the role of this multigene family in plant development and adaptation to environmental stimuli.

Keywords: biotic and abiotic stress; calcium signaling; calmodulin; calmodulin-binding proteins; calmodulin-like protein; plant development.

Figure 1

Neighbour joining tree based on amino acid similarities of CaM and CML proteins from different plant species. Sequences of CaM and CML proteins were downloaded from SwissProt or from the TIGR and subjected to phylogenetic analysis using MEGA version 3.1 software. Alignments of CaM and CML proteins ranging from 150 to 200 amino acids and harbouring 4 EF-hand from different plant species were constructed using the multiple sequence alignment mode of ClustalW with protein matrix blosum. Protein tree was constructed by the neighbour-joining method implemented in the MEGA 3.1 program. The distance indicated by ‘0.1’ refers to the percent sequence divergence as calculated by ClustalW. AtCaM/AtCML refer to Arabidopsis thaliana CaM or CML (♦); Rice calmodulins (▴) are named OsCaM for Oryza sativa CaM or according to their TIGR accession; NtCam: Nicotiana tabacum CaM (▾); MtCaM: Medicago truncatula CaM (•); GmCaM or Scam refer to Glycine max or soybean CaM (▪); HsCaM: Homo sapiens CaM (P62158) (○); DmCaM: Drosophila melanogaster CaM (P62152) (○). The percentage of identity is indicated by using AtCaM2 as reference.

Figure 2

Calcium signaling with CaMs and CMLs. In response to environmental and developmental stimuli, a transient elevation of Ca²⁺ concentration can be observed in the cytosol as well as in other cellular compartments. These Ca²⁺ variations can be decoded by a wide range of Ca²⁺ sensors such as the typical CaM and CMLs proteins which are highly abundant in plant cell. Some of these proteins have been shown to interact and modulate the activity of downstream target proteins to relay the Ca²⁺ message and to initiate biochemical, cellular and physiological responses. Many of these targets have been identified as typical CaM binding proteins and in most cases their interaction with CMLs need to be examined.