CDPK versus calmodulin: a rivalry in Ca2+ pump control

In plants, Ca2+ is an important second messenger for the responses to various signals, such as hormones, light, oxidative stress, drought, cold, and pathogen elicitors. Specific responses to these different signals are achieved through variations in the amplitude, duration, location and frequency of the ‘spikes’ Ca2+ concentration. Traditionally, the key point of Ca2+ signal regulation has been viewed as the influx of Ca2+ through channels, whereas Ca2+ efflux through pumps or H+/Ca2+ antiporters has been viewed simply as necessary for resetting the basal Ca2+ level. Work by Ildoo Hwang et al. with the Arabidopsis Ca2+ pump ACA2 [located in the endoplasmic reticulum (ER)] suggests that Ca2+ efflux is tightly controlled by two rival Ca2+ sensors. Thus, the regulation of Ca2+ efflux could play a more active role in plant signal transduction than previously thought.Hwang et al.1xA calcium-dependent protein kinase can inhibit a calmodulin-stimulated Ca2+ pump (ACA2) located in the endoplasmic reticulum of Arabidopsis. Hwang, I. et al. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 6224–6229Crossref | PubMed | Scopus (121)See all References1 show that CPK1, a Ca2+-dependent protein kinase (CDPK), can phosphorylate ACA2 in vitro within the N-terminal autoinhibitory region at Ser45. It has been shown that the animal homologs of ACA2, the plasma membrane Ca2+-ATPases (PCMAs), are mostly activated by phosphorylation. Suprisingly, phosphorylation of Ser45 inhibited the basal as well as the calmodulin-stimulated Ca2+ efflux activities of ACA2. Phosphorylation of ACA2 does not prevent the binding of its activator calmodulin, although calmodulin binding can inhibit phosphorylation. Ca2+ efflux by ACA2 depends, therefore, upon the balance of Ca2+-activated calmodulin and CDPK. These two Ca2+ sensors probably have different affinities for Ca2+. Hwang et al. hypothesize that the CDPK phosphorylation inhibits Ca2+ efflux by stabilizing ACA2 in an autoinhibited conformation, allowing for a greater accumulation of Ca2+ within the cells. When the Ca2+ concentration is high enough, calmodulin is probably activated and binds to ACA2, which prevents ACA2 phosphorylation by CDPK, allowing for an increase in Ca2+ efflux and a return to the steady-state condition. The researchers postulate that calmodulin binding of ACA2 converts the protein to an activated conformation that is resistant to phosphorylation. Future in vivo experiments could test the localization and specificity of CPK1 compared with the other 40 CDPKs. Although CPK1 has a myristoylation sequence, suggesting it is membrane-associated, it is not known if it is localized and oriented properly to phosphorylate the N-terminus of ACA2 in the cytoplasm. ACA2 is the first Ca2+ transport protein in plants that has been shown to be regulated by phosphorylation and multiple Ca2+ sensors.The tight control of ACA2 suggests that Ca2+ efflux could be an important point of Ca2+ signal regulation. Changing the timing, position and rate of Ca2+ efflux could allow for variations in the characteristics of Ca2+ spikes and, in turn, the specific responses elicited. Ca2+ fluxes across the ER membrane are probably crucial for Ca2+-mediated signal transduction in plant cells because Arabidopsis actually has two known differentially regulated ER-localized Ca2+ pumps.