Characterization of vacuolar calcium-binding proteins

Are calcium oxalate crystals a dynamic calcium store in plants?

Calcium oxalate (CaOx) crystals occur as intravacuolar deposits in most angiosperm species. Different functions have been attributed to these crystals, some of which are very speculative, until now. Calcium regulation and homeostasis seem to be the most widespread function of CaOx crystals. Being rich in calcium, these crystals constitute a reserve of calcium for plants. However, despite being bioavailable, this reserve is functional in just a few situations due to the low mobility of calcium for phloem translocation. Therefore, CaOx crystals as a calcium reserve is a paradox because in most cases the reserve cannot be used. However, in most plants, these crystals occur in organs or tissues that will be discarded, which allows the elimination of excess calcium. This suggests that CaOx crystals have a functional role in excess calcium excretion. There is some evidence that, for calcium, this excretory function is relevant for plants since they lack an excretory system dedicated to discarding solid wastes, such as calcium salts.

The calcium-binding activity of a vacuole-associated, dehydrin-like protein is regulated by phosphorylation

A vacuole membrane-associated calcium-binding protein with an apparent mass of 45 kD was purified from celery (Apium graveolens). This protein, VCaB45, is enriched in highly vacuolate tissues and is located within the lumen of vacuoles. Antigenically related proteins are present in many dicotyledonous plants. VCaB45 contains significant amino acid identity with the dehydrin family signature motif, is antigenically related to dehydrins, and has a variety of biochemical properties similar to dehydrins. VCaB45 migrates anomalously in sodium dodecyl sulfate-polyacrylamide gel electrophoresis having an apparent molecular mass of 45 kD. The true mass as determined by matrix-assisted laser-desorption ionization time of flight was 16.45 kD. VCaB45 has two characteristic dissociation constants for calcium of 0.22 +/- 0.142 mM and 0.64 +/- 0.08 mM, and has an estimated 24.7 +/- 11.7 calcium-binding sites per protein. The calcium-binding properties of VCaB45 are modulated by phosphorylation; the phosphorylated protein binds up to 100-fold more calcium than the dephosphorylated protein. VCaB45 is an "in vitro" substrate of casein kinase II (a ubiquitous eukaryotic kinase), the phosphorylation resulting in a partial activation of calcium-binding activity. The vacuole localization, calcium binding, and phosphorylation of VCaB45 suggest potential functions.

A Vacuole-Associated Annexin Protein, VCaB42, Correlates with the Expansion of Tobacco Cells

A Ca-dependent membrane-binding protein of the annexin family, VCaB42, has previously been shown to associate with vacuolar vesicles at physiological levels of Ca. In this study we used suspension-cultured cells of tobacco (Nicotiana tabacum BY-2) to show that VCaB42 is enriched 4.5-fold in intact vacuoles, whereas evacuolated protoplasts show a 12-fold reduction in VCaB42. VCaB42 distribution is thus comparable to that of the vacuole-associated H+-ATPase but is distinct from the endoplasmic reticulum-localized protein calnexin. Because VCaB42 is a vacuole-associated annexin, and given the putative function of annexins in vesicle fusion, we hypothesize a role for this protein in the vacuolation process of expanding cells. Consistent with this hypothesis, we show that VCaB42 levels correlate with age-associated and hormonally induced changes in cell volume in tobacco suspension cultures. The association of VCaB42 with vacuoles and its correlative pattern of expression relative to the expansion of cells is consistent with a possible role for VCaB42 in the early events of vacuole biogenesis.

Electrochemical Potential Gradients of H+, K+, Ca2+, and Cl- across the Tonoplast of the Green Alga Eremosphaera Viridis

Using ion-selective microelectrodes, we measured the activity of H+, K+, Ca2+, and Cl- and the electrical potential both in the vacuole and in the cytoplasm of the unicellular green alga Eremosphaera viridis to obtain comparable values of the named parameters from the same object under identical conditions. The cytosol had a pH of 7.3, and activities of the other ions were 130 mM K+, 160 nM Ca2+, and 2.2 mM Cl-. We observed only small and transient light-dependent changes of the cytosolic Ca2+ activity. The vacuolar K+ activity did not differ significantly from the cytosolic one. The Ca2+ activity inside the vacuole was approximately 200 [mu]M, the pH was 5.0, and the Cl- activity was 6.2 mM. The concentrations of K+, Ca2+, and Cl- in cell extracts were measured by induction-coupled plasma spectroscopy and anion chromatography. This confirmed the vacuolar activities for K+ and Cl- obtained with ion-selective microelectrodes and indicated that approximately 60% of the vacuolar Ca2+ was buffered. The tonoplast potential was vanishingly low ([less than or equal to][plus or minus]2 mV). There was no detectable electrochemical potential gradient for K+ across the tonoplast, but there was, however, an obvious electrochemical potential gradient for Cl- (-26 mV), indicating an active accumulation of Cl- inside the vacuole.

A 42-kilodalton annexin-like protein is associated with plant vacuoles

A 42-kD, calcium-dependent, membrane-binding protein (VCaB42) was associated with partially purified vacuole membrane. Membrane-dissociation assays indicated that VCaB42 binding to vacuole membranes was selective for calcium over other cations and that 50% of VCaB42 remained membrane bound at 61 +/- 11 nM free calcium. A 13-amino acid sequence obtained from VCaB42 showed 85% similarity with the endonexin fold, a sequence found in the annexin family of proteins that is thought to be essential for calcium and lipid binding. The greatest similarity in amino acid sequence was observed with annexin VIII (VAC-beta). The calcium-binding properties and sequence similarities suggest that VCaB42 is a member of the annexin family of calcium-dependent, membrane-binding proteins. Functional assays for VCaB42 on vacuole membrane transport processes indicated that it did not significantly affect the initial rate of calcium uptake into vacuole membrane vesicles. Because VCaB42 is vacuole localized (likely on the cytosolic surface of the vacuole) and is 50% dissociated within the physiological range of cytosolic free calcium, we hypothesize that this protein is a sensor that monitors cytosolic calcium levels and transmits that information to the vacuole.