Detection of calcium binding proteins by two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis

Salt stress triggers phosphorylation of the Arabidopsis vacuolar K+ channel TPK1 by calcium-dependent protein kinases (CDPKs)

14-3-3 proteins play an important role in the regulation of many cellular processes. The Arabidopsis vacuolar two-pore K(+) channel 1 (TPK1) interacts with the 14-3-3 protein GRF6 (GF14-λ). Upon phosphorylation of the putative binding motif in the N-terminus of TPK1, GRF6 binds to TPK1 and activates the potassium channel. In order to gain a deeper understanding of this 14-3-3-mediated signal transduction, we set out to identify the respective kinases, which regulate the phosphorylation status of the 14-3-3 binding motif in TPK1. Here, we report that the calcium-dependent protein kinases (CDPKs) can phosphorylate and thereby activate the 14-3-3 binding motif in TPK1. Focusing on the stress-activated kinase CPK3, we visualized direct and specific interaction of TPK1 with the kinase at the tonoplast in vivo. In line with its proposed role in K(+) homeostasis, TPK1 phosphorylation was found to be induced by salt stress in planta, and both cpk3 and tpk1 mutants displayed salt-sensitive phenotypes. Molecular modeling of the TPK1-CPK3 interaction domain provided mechanistic insights into TPK1 stress-regulated phosphorylation responses and pinpointed two arginine residues in the N-terminal 14-3-3 binding motif in TPK1 critical for kinase interaction. Taken together, our studies provide evidence for an essential role of the vacuolar potassium channel TPK1 in salt-stress adaptation as a target of calcium-regulated stress signaling pathways involving Ca(2+), Ca(2+)-dependent kinases, and 14-3-3 proteins.

Molecular sabotage of plant defense by aphid saliva

Aphids, which constitute one of the most important groups of agricultural pests, ingest nutrients from sieve tubes, the photoassimilate transport conduits in plants. Aphids are able to successfully puncture sieve tubes with their piercing mouthparts (stylets) and ingest phloem sap without eliciting the sieve tubes' normal occlusion response to injury. Occlusion mechanisms are calcium-triggered and may be prevented by chemical constituents in aphid saliva injected into sieve tubes before and during feeding. We recorded aphid feeding behavior with the electrical penetration graph (EPG) technique and then experimentally induced sieve tube plugging. Initiation of sieve tube occlusion caused a change in aphid behavior from phloem sap ingestion to secretion of watery saliva. Direct proof of "unplugging" properties of aphid saliva was provided by the effect of aphid saliva on forisomes. Forisomes are proteinaceous inclusions in sieve tubes of legumes that show calcium-regulated changes in conformation between a contracted state (below calcium threshold) that does not occlude the sieve tubes and a dispersed state (above calcium threshold) that occludes the sieve tubes. We demonstrated in vitro that aphid saliva induces dispersed forisomes to revert back to the nonplugging contracted state. Labeling Western-blotted saliva proteins with 45Ca2+ or ruthenium red inferred the presence of calcium-binding domains. These results demonstrate that aphid saliva has the ability to prevent sieve tube plugging by molecular interactions between salivary proteins and calcium. This provides aphids with access to a continuous flow of phloem sap and is a critical adaptation instrumental in the evolutionary success of aphids.

Assays of RGS3 activation and modulation

Regulator of G-protein signaling (RGS) proteins accelerate the intrinsic GTPase activity of Galpha subunits, terminating G-protein signaling. Although the structure and the downstream effectors of RGS proteins have been investigated extensively, the mechanisms underlying their activation are still largely unexplored. Recent investigations of RGS proteins in intact cells are starting to shed some light on this issue. In particular, the retrovirus-mediated overexpression of individual RGS3 isoforms showed that activation of RGS3s requires Ca2+ binding to an EF-like domain, and such Ca2+ binding promotes the rapid desensitization of Go-mediated inhibition of N-type Ca2+ (N) channels. This article provides detailed information on the use and power of retrovirus-mediated overexpression of RGS proteins in dorsal root ganglion neurons to isolate and identify the mechanisms underlying the initiation of RGS3-dependent inhibitory activity.

Mitochondria are intracellular magnesium stores: investigation by simultaneous fluorescent imagings in PC12 cells

To determine the nature of intracellular Mg2+ stores and Mg2+ release mechanisms in differentiated PC12 cells, Mg2+ and Ca2+ mobilizations were measured simultaneously in living cells with KMG-104, a fluorescent Mg2+ indicator, and fura-2, respectively. Treatment with the mitochondrial uncoupler, carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP), increased both the intracellular Mg2+ concentration ([Mg2+]i) and the [Ca2+]i in these cells. Possible candidates as intracellular Mg2+ stores under these conditions include intracellular divalent cation binding sites, endoplasmic reticulum (ER), Mg-ATP and mitochondria. Given that no change in [Mg2+]i was induced by caffeine application, intracellular IP3 or Ca2+ liberated by photolysis, it appears that no Mg2+ release mechanism thus exists that is mediated via the action of Ca2+ on membrane-bound receptors in the ER or via the offloading of Mg2+ from binding sites as a result of the increased [Ca2+]i. FCCP treatment for 2 min did not alter the intracellular ATP content, indicating that Mg2+ was not released from Mg-ATP, at least in the first 2 min following exposure to FCCP. FCCP-induced [Mg2+]i increase was observed at mitochondria localized area, and vice versa. These results suggest that the mitochondria serve as the intracellular Mg2+ store in PC12 cell. Simultaneous measurements of [Ca2+]i and mitochondrial membrane potential, and also of [Ca2+]i and [Mg2+]i, revealed that the initial rise in [Mg2+]i followed that of mitochondrial depolarization for several seconds. These findings show that the source of Mg2+ in the FCCP-induced [Mg2+]i increase in PC12 cells is mitochondria, and that mitochondrial depolarization triggers the Mg2+ release.

Proteomic identification of divalent metal cation binding proteins in plant mitochondria

Divalent metal binding proteins in the Arabidopsis mitochondrial proteome were analysed by mobility shifts in the presence of divalent cations during two-dimensional diagonal sodium dodecyl sulphate-polyacrylamide gel electrophoresis. Tandem mass spectrometry and searches of the predicted Arabidopsis protein dataset were used in an attempt to identify 34 of the proteins which shifted. This analysis identified a total of 23 distinct protein spots as the products of at least 11 different Arabidopsis genes. A series of proteins known to be divalent cation-binding proteins, or to catalyse divalent cation-dependent reactions, were identified. These included: succinyl CoA ligase beta subunit, Mn-superoxide dismutase (SOD), an Fe-S centred component of complex I and the REISKE iron-sulphur protein of the b/c(1) complex. A further set of four proteins of known function but without known divalent binding properties were also identified: the Vb subunit of cytochrome c oxidase, a subunit of ATP synthase (orfB), the acyl carrier protein, and the translocase of the outer membrane (TOM20). Three other proteins, of unknown function, were also found to shift in the presence of divalent cations. This approach has broad application for the identification of sub-proteomes based on the metal interaction of polypeptides.

Mobility shift assay of calcium-binding proteins of mouse epididymal spermatozoa

The calcium-binding proteins (CBPs) of mouse epididymal spermatozoa were analysed by mobility changes in the presence of added Ca2+ in two-dimensional sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). The expression patterns of relatively high molecular weight CBPs (Mr > 20 kDa) were different between caput and cauda epididymal spermatozoa. There was a constitutive expression of low molecular weight CBPs (Mr < 20 kDa) regardless of the epididymal region. Most of the CBPs disappeared after the acrosome reaction (AR) induced by Ca2+ ionophore A23187, suggesting that they originated from the acrosome and/or the plasma membrane overlaying the acrosome. Taken together, it can be suggested that changes in CBPs of spermatozoa are important features of sperm maturation during epididymal transit, and that they may be related to the fertilizing ability of mouse epididymal spermatozoa.