Structurally, Ca 2 +-ATPases are single polypeptide (1000–1100 amino acids) with 10 transmembrane domains ( Fig. Plant Ca 2 + ATPases belong to the P-type ATPase superfamily that has characteristic feature of phosphorylated intermediate and presence of several conserved domains. The molecular mechanism of divalent cation transport and affinity for Ca 2+ is yet to determine in ECAs and ACAs of plants. ACAs are specifically inhibited by fluorescein derivatives such as erythrosin B ( Sze et al., 2000 Geisler et al., 2000a Bonza and De Michelis, 2011). The ACAs have affinity for Ca 2+ in micromolar range and are selective and highly specific in Ca 2+ transport ( Bonza and De Michelis, 2011 Bonza et al., 2001 Huda et al., 2013b). The binding of CaM relieves autoinhibitory constrains after binding to the autoinhibitory domain, which overlaps partially with the CMBD domain ( Baekgaard et al., 2006).
Plants ACAs have N-terminal CaM-binding domain (CMBD) while it is present at the C-terminus in animal PMCAs ( Sze et al., 2000 James et al., 1988). They are activated by binding of CaM protein to autoinhibitory domain in the presence of Ca 2+ in plants ( Malmström et al., 1997 Harper et al., 1998 Hwang et al., 2000). The plant P-IIB Ca 2+-ATPases are autoinhibited Ca 2+-ATPases (ACAs) and are similar to animal PMCAs. The cyclopiazonic acid (CPA an inhibitor of IIA Ca 2+-ATPase) specifically inhibits the activity of ECAs ( Liang and Sze, 1998 Iwano et al., 2009). The ECAs have affinity for Ca 2+ in submicromolar range and transport Ca 2+ along with other divalent cations such as Cd 2+, Mn 2+, and Zn 2+ ( Bonza and De Michelis, 2011 Bonza et al., 2001 Huda et al., 2013b).
The animal SERCAs release Ca 2+ from and into the sarcoplasmic reticulum of muscle cells during muscle contraction and relaxation, respectively ( Palmgren et al., 2011 Berchtold et al., 2000). The plant P-IIA Ca 2+-ATPases are endoplasmic reticulum Ca 2+-ATPases (ER-type Ca 2+-ATPase, ECAs) show structural as well as transport similarity with animal SERCAs ( Bonza and De Michelis, 2011). The plant Ca 2+-ATPases are members of P-type superfamily and is further divided into two phylogenetic subgroups as P-IIA (P2A) and P-IIB (P2B) ( Møller et al., 1996 Axelsen and Palmgren, 1998). The plant Ca 2+-ATPases show similarity with animal Ca 2+-ATPases in their structure and Ca 2+ transport activity. Plants genome encode several Ca 2+-ATPases, and phylogenetic analysis revealed that plant Ca 2+-ATPases align with sarcoplasmic/endoplasmic reticulum Ca 2+-ATPases (SERCA) or with the PM Ca 2+-ATPases (PMCA) of animals ( Geisler et al., 2000a Baxter et al., 2003). The Ca 2+-ATPases play important role in Ca 2+ efflux across the cellular membranes for proper Ca 2+ homeostasis in the plant cells. Akhilesh Kumar Yadav, in Calcium Transport Elements in Plants, 2021 7.3 Plant Ca 2+-ATPase: classification
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