1.A.87 The Mechanosensitive Calcium Channel (MCA) Family

Mechano-sensitive channels of plants sense increases in tension induced by mechanical stimuli, such as touch, wind, turgor pressure and gravitation. Plant homologues of MscS bacterial mechano-sensitive channels are known which are gated by membrane tension. Two of them have been shown to be involved in the protection of osmotically stressed plastids in Arabidopsis thaliana (see TC# 1.A.23.4.4). Membrane tension is not a mediator of long-range intracellular signaling, but local variations in tension mediate distinct processes in sub-cellular domains (Shi et al. 2018). Lipid bilayer tensiometers have been used for the study of mechanosensitive ion channels (Pérez-Mitta and MacKinnon 2023).

Iida et al. (2013) identified another group of candidates for mechano-sensitive channels in Arabidopsis, named MCA1 and MCA2, whose homologues are exclusively found in plant genomes. MCA1 and MCA2 are composed of 421 and 416 amino acyl residues, respectively, share 73% identity in their amino acid sequences, and are not homologous to any other known ion channels or transporters. A structural study revealed that the N-terminal region (~173 amino acids) of both proteins is necessary and sufficient for Ca²⁺ influx activity. This region has one putative transmembrane segment containing an Asp residue whose substitution mutation abolished activity.Their physiological study suggested that MCA1, expressed at the root tip, is required for sensing the hardness of the agar medium or soil. In addition, MCA1 and MCA2 were shown to be responsible for hypo-osmotic shock-induced increases in [Ca²⁺]_cyt . Thus, both proteins appear to be involved in the process of sensing mechanical stresses. Iida et al. (2013) discussed the possible roles of both proteins in sensing mechanical and gravitational stimuli.  Several homologues may serve as receptors and regulatory proteins rather than ion channels, and several of these are included in this family in TCDB.  Their roles as mechanosensitive plasma membrane Ca²⁺-permeable channels, such as OsMCA1and OsMCA2 in rice seems to allow them to play roles in the generation of reactive oxygen species and in hypo-osmotic signaling (Kurusu et al. 2012; Kurusu et al. 2012; Kurusu et al. 2012).

MCA proteins show various topologies.  Several show a 1 + 3 TMS topology (subfamily 1) while others (subfamily 2) appear to have a 1 + 3 + 3 TMS topology, and still others have just 3 TMSs (subfamily 3).  The 3 TMSs in these last mentioned proteins appear to correspond to the last 3 TMSs in subfamilies 1 and 2. The topologies of subfamilies 4 and 5 are not clear.  There may be additional topological variations.

The FW2.2 gene is associated with the major Quantitative Trait Locus (QTL) governing fruit size in the tomato, and it acts by negatively controlling cell division during fruit development. FW2.2 belongs to a multigene family named the CELL NUMBER REGULATOR (CNR) family (Beauchet et al. 2021). The CNR proteins harbour the uncharacterized PLAC8 motif made of two conserved cysteine-rich domains separated by a variable region that are predicted to be transmembrane segments, and indeed FW2.2 localizes to the plasma membrane.  Beauchet et al. 2021 reviewed the knowledge on PLAC8-containing CNR/FWL proteins in plants, which participate in plant organogenesis and the regulation of organ size, especially in fruits, and in cadmium resistance, ion homeostasis and/or Ca²⁺ signalling. Within the plasma membrane, FW2.2 and some CNR/FWL proteins are localized in microdomains. Hence FW2.2 and CNR/FWL could be involved in a transport function of signalling molecules across membranes, thus influencing organ growth via a cell-to-cell trafficking mechanism (Beauchet et al. 2021).

The generalized reaction reported to be catalyzed by MCA1 and MCA2 is:

Ca²⁺(out)  →  Ca²⁺ (in)