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).
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 generalized reaction reported to be catalyzed by MCA1 and MCA2 is:
Ca2+(out) → Ca2+ (in)
Plant Ca2+ channel protein, Mid1 complementary activity 1, MCA1 (Iida et al. 2013). MCA1 and MCA2 each forms a homotetramer and exhibit Ca2+-permeable MS channel activity. Both are single-pass type I transmembrane proteins with their N-termini located extracellularly and their C-termini located intracellularly. An EF hand-like motif, coiled-coil motif, and Plac8 motif may all be in the cytoplasm, suggesting that the activities of both channels can be regulated by intracellular Ca2+ and protein interactions (Kamano et al. 2015). However, hydropathy plots suggest that the Plac8 domain may be transmembrane with 3 TMSs. mca1 but not mca2 mutants show defects in root entry into hard agar, whereas mca2 but not mca1 mutants are defective in Ca2+ uptake in A. thaliana roots (Hamilton et al. 2015).
MCA1 of Arabidopsis thaliana
Plant Ca2+ channel protein, Mid1 complementary activity 2, MCA2 (Iida et al. 2013). Catalyzes mechanical stress-induced Ca2+ influx. It is tetrameric with a small transmembrane domain and a large cytoplasmic domain (Shigematsu et al. 2014). MCA1 and MCA2 both have their N-termini located extracellularly and their C-termini located intracellularly. An EF hand-like motif, coiled-coil motif, and Plac8 motif may all be in the cytoplasm, suggesting that the activities of both channels can be regulated by intracellular Ca2+ and protein interactions (Kamano et al. 2015). However hydropathy plots suggest that the Plac8 domain may be transmembrane with 3 TMSs. mca1 but not mca2 mutants show defects in root entry into hard agar, whereas mca2 but not mca1 mutants are defective in Ca2+ uptake in A. thaliana roots (Hamilton et al. 2015).
MCA2 of Arabidopsis thaliana
MCA1 isoform X2 of 377 aas
MCA1 of Solanum pennellii (Lycopersicon pennellii)
PLAC8 family protein of 385 aas.
PLAC8 family protein of Theobroma cacao
Mid1 complementing activity 1 of 154 aa
MCA1 of Vigna radiata
Receptor protein kinase of 567 aas. The first 140 aas are homologous to the N-terminal domains of MCA1 and 2; residues 240 - 430 are homologous to ser/thr protein kinases of 9.A.15.1.1, 9.B.45.1.3 and 9.B.106.3.1.
Receptor protein kinase of Zea mays
Protein kinase domain protein of 522 aas.
PKD protein of Oryza sativa
Receptor for extracellular ATP which functions in plant growth, development and stress responses; lectin receptor kinase 1.9; DORN1. Binds ATP with high affinity (46nM) and is required ofr ATP-induced calcium response, mitogen-activated protein kinase activation and normal gene expression (Choi et al. 2014).
DORN1 of Arabidopsis thaliana
GHR1 (GUARD CELL HYDROGEN PEROXIDE-RESISTANT 1) transmembrane receptor-like protein of 1053 aas and 1 - 3 TMSs. Regulates the SLAC1 protein (2.A.16.5.1) (Wang et al. 2017). The C-terminus shows extensive sequence similarity with members of this family, but the N-terminus shows similarity with members of family 3.A.20 (Leucine repeat proteins).
GHR1 of Arabidopsis thaliana
Uncharacterized protein with an ATP binding domain of 629 aas and 2 TMSs.
UP of Arabidopsis thaliana
Protein BRASSINOSTEROID INSENSITIVE 1, BRI1, of 1196 aas and 2 or 3 TMSs. Receptor with kinase activity
acting on both serine/threonine- and tyrosine-containing substrates. In response to brassinosteroid binding, it regulates a signaling cascade
involved in plant development, including expression of light- and
stress-regulated genes, promotion of cell elongation, normal leaf and
chloroplast senescence, and flowering. It binds brassinolide, and less
effectively, castasterone (Oh et al. 2009).
BRI1 of Arabidopsis thaliana
Plant cadmium resistance, PCR, protein of 164 aas. Shows homology to the C-terminal PLAC8 domain of MCA1 and 2.
Cadmium resistance protein of Solanum lycopersicum (Tomato) (Lycopersicon esculentum)
Fruit-weight 2.2 protein of 197 aas and 3 TMSs. May be involved in Cd2+ resistance as well as translocation of Cd2+ from roots to shoots (Xiong et al. 2018). May form homooligomeric structures in the membrane.
FWL protein of Medicago truncatula (Barrel medic) (Medicago tribuloides)
Fruit-weight 2.2 protein of 161 aas and 4 TMSs. May be involved in Cd2+ resistance as well as translocation of Cd2+ from roots to shoots (Xiong et al. 2018). May form homooligomeric structures in the membrane.
FWL protein of Medicago truncatula (Barrel medic) (Medicago tribuloides)
Plant Cadmium Resistance (PCR) protein. This protein corresponds to the C-terminal PLAC8 domain of MCA1 (TC# 1.A.87.1.1) (Song et al., 2011).
PLAC8 family protein of Arabidopsis thaliana
Sea squirt membrane protein of 110 aas
Membrane protein of Ciona intestinalis
Uncharacterized protein of 161 aas
UP of Capsella rubella
Plant cadmium resistance 6 protein, CadR6, of 224 aas.
CadR6 of Arabidopsis thaliana
Uncharacterized protein of 186 aas
UP of Glycine max
Plant cadmium resistance 1 protein of 151 aas and 2 TMSs. PCR1. Involved in glutathione-independent cadmium resistance. Reduces cadmium uptake rather than activating efflux, but is not closely coupled to calcium transport (Song et al. 2011).
PCR1 of Arabidopsis thaliana
Plant cadmium resistance 2 (PCR2) protein. Zinc ion exporter (Song et al. 2010; Song et al. 2011). Involved in glutathione-independent cadmium resistance. Reduces cadmium uptake rather than activating efflux, but is not closely coupled to calcium transport.
PCR2 of Arabidopsis thaliana
FW2.2-like (FWL) protein of 180 aas and 2 TMSs. Involved in plant and fruit development, and possibly in calcium transport (Libault and Stacey 2010).
FWL of Persea americanan (Avocado)
Ubiquitin protein ligase with the first 250 aas homologous to MCA2.
Ubiquitin ligase of Physcomitrella patens
U box containing protein 15
U box protein of Solanum lycopersicum (Tomato) (Lycopersicon esculentum)
Protein kinase_Tyr of 657 aas with N-terminal domain similar to that of MCA1, with N-terminal TMS containing a conserved aspartyl residue.
PKinase-Tyr of Phanerochaete carnosa