Guard cells undergo constitutive and pressure-driven membrane turnover

Summary. During stomatal movement, guard cells undergo large and reversible changes in cell volume and consequently surface area. These alterations in surface area require addition and removal of plasma membrane material. How this is achieved is largely unknown. Here we summarize recent studies of membrane turnover in guard cells using electrophysiology and fluorescent imaging techniques. The results implicate that membrane turnover in guard cells and most likely in plant cells in general is sensitive to changes in membrane tension. We suggest that this provides a mechanism for the adaptation of surface area of guard cells to osmotically driven changes in cell volume. In addition, guard cells also exhibit constitutive membrane turnover. Constitutive and pressure-driven membrane turnover were found to be associated with addition and removal of K⁺ channels. This implies that some of the exo- and endocytic vesicles carry K⁺ channels. Together the results demonstrate that exo- and endocytosis is an essential process in guard cell functioning. Correspondence and reprints: Institute of Botany, Darmstadt University of Technology, Schnittspahnstrasse 3, 64287 Darmstadt, Federal Republic of Germany.     

Stabilization of non-productive conformations underpins rapid electron transfer to electron-transferring flavoprotein

Crystal structures of protein complexes with electron-transferring flavoprotein (ETF) have revealed a dual protein-protein interface with one region serving as anchor while the ETF FAD domain samples available space within the complex. We show that mutation of the conserved Glu-165beta in human ETF leads to drastically modulated rates of interprotein electron transfer with both medium chain acyl-CoA dehydrogenase and dimethylglycine dehydrogenase. The crystal structure of free E165betaA ETF is essentially identical to that of wild-type ETF, but the crystal structure of the E165betaA ETF.medium chain acyl-CoA dehydrogenase complex reveals clear electron density for the FAD domain in a position optimal for fast interprotein electron transfer. Based on our observations, we present a dynamic multistate model for conformational sampling that for the wild-type ETF. medium chain acyl-CoA dehydrogenase complex involves random motion between three distinct positions for the ETF FAD domain. ETF Glu-165beta plays a key role in stabilizing positions incompatible with fast interprotein electron transfer, thus ensuring high rates of complex dissociation.     

Essential and nonessential elements in the 3' nontranslated region of Bovine viral diarrhea virus

The 3' nontranslated region (NTR) of the pestivirus Bovine viral diarrhea virus (BVDV), a close relative of human Hepatitis C virus, consists of three stem-loops which are separated by two single-stranded regions. As in other positive-stranded RNA viruses, the 3' NTR of pestiviruses is involved in crucial processes of the viral life cycle. While several studies characterized cis-acting elements within the 3' NTR of a BVDV replicon, there are no studies addressing the significance of these elements in the context of a replicating virus. To examine the functional importance of 3' NTR elements, a set of 4-base deletions and deletions of each of the three stem-loops were introduced into an infectious BVDV cDNA clone. Emerging mutant viruses were characterized with regard to plaque phenotype, growth kinetics, and synthesis of viral RNA. The results indicated that presence of stem-loop (SL) I and the 3'-terminal part of the single-stranded region between stem-loops I and II are indispensable for pestiviral replication. In contrast, deletions within SL II and SL III as well as absence of either SL II or SL III still allowed efficient viral replication; however, a mutant RNA lacking both SL II and SL III was not infectious. The results of this study provide a detailed map of the essential and nonessential elements within the 3' NTR of BVDV and contribute to our understanding of sequence and structural elements important for efficient viral replication of pestiviruses in natural host cells.     

The viral potassium channel Kcv: structural and functional features

The chlorella virus PBCV-1 was the first virus found to encode a functional potassium channel protein (Kcv). Kcv is small (94 aa) and basically consists of the M1-P-M2 (membrane-pore-membrane) module typical of the pore regions of all known potassium channels. Kcv forms functional channels in three heterologous systems. This brief review discusses the gating, permeability and modulation properties of Kcv and compares them to the properties of bacterial and mammalian K+ channels.     

Cholinergic modulation of experience-dependent plasticity in human auditory cortex

The factors that influence experience-dependent plasticity in the human brain are unknown. We used event-related functional magnetic resonance imaging (fMRI) and a pharmacological manipulation to measure cholinergic modulation of experience-dependent plasticity in human auditory cortex. In a differential aversive conditioning paradigm, subjects were presented with high (1600 Hz) and low tones (400 Hz), one of which was conditioned by pairing with an electrical shock. Prior to presentation, subjects were given either a placebo or an anticholinergic drug (0.4 mg iv scopolamine). Experience-dependent plasticity, expressed as a conditioning-specific enhanced BOLD response, was evident in auditory cortex in the placebo group, but not with scopolamine. This study provides in vivo evidence that experience-dependent plasticity, evident in hemodynamic changes in human auditory cortex, is modulated by acetylcholine.     

Phylogenetic identification of bacteria with antimicrobial activities isolated from Mediterranean sponges

Bacteria were isolated from Mediterranean sponges and specifically selected according to their antimicrobial activity. They were classified on the basis of partial 16S rDNA sequences as members of distinct clusters of alpha-Proteobacteria and gamma-Proteobacteria. The action spectrum of antimicrobial activities against target organisms was related to the phylogenetic position of the isolates.