Pathways followed by ricin and Shiga toxin into cells

The plant toxin ricin and the bacterial toxin Shiga toxin belong to a group of protein toxins that inhibit protein synthesis in cells enzymatically after entry into the cytosol. Ricin and Shiga toxin, which both have an enzymatically active moiety that inactivates ribosomes and a moiety that binds to cell surface receptors, enter the cytosol after binding to the cell surface, endocytosis by different mechanisms, and retrograde transport to the Golgi apparatus and the endoplasmic reticulum (ER). The toxins can be used to investigate the various transport steps involved, both the endocytic mechanisms as well as pathways for retrograde transport to the ER. Recent studies show that not only do several endocytic mechanisms exist in the same cell, but they are not equally sensitive to removal of cholesterol. New data have revealed that there is also more than one pathway leading from endosomes to the Golgi apparatus and retrogradely from the Golgi to the ER. Trafficking of protein toxins along these pathways will be discussed in the present article.     

Jasmonate-induced lipid peroxidation in barley leaves initiated by distinct 13-LOX forms of chloroplasts

In addition to a previously characterized 13-lipoxygenase of 100 kDa encoded by LOX2:Hv:1 [V?r?s et al., Eur. J. Biochem. 251 (1998), 36-44], two full-length cDNAs (LOX2:Hv:2, LOX2:Hv:3) were isolated from barley leaves (Hordeum vulgare cv. Salome) and characterized. Both of them encode 13-lipoxygenases with putative target sequences for chloroplast import. Immunogold labeling revealed preferential, if not exclusive, localization of lipoxygenase proteins in the stroma. The ultrastructure of the chloroplast was dramatically altered following methyl jasmonate treatment, indicated by a loss of thylakoid membranes, decreased number of stacks and appearance of numerous osmiophilic globuli. The three 13-lipoxygenases are differentially expressed during treatment with jasmonate, salicylate, glucose or sorbitol. Metabolite profiling of free linolenic acid and free linoleic acid, the substrates of lipoxygenases, in water floated or jasmonate-treated leaves revealed preferential accumulation of linolenic acid. Remarkable amounts of free 9- as well as 13-hydroperoxy linolenic acid were found. In addition, metabolites of these hydroperoxides, such as the hydroxy derivatives and the respective aldehydes, appeared following methyl jasmonate treatment. These findings were substantiated by metabolite profiling of isolated chloroplasts, and subfractions including the envelope, the stroma and the thylakoids, indicating a preferential occurrence of lipoxygenase-derived products in the stroma and in the envelope. These data revealed jasmonate-induced activation of the hydroperoxide lyase and reductase branch within the lipoxygenase pathway and suggest differential activity of the three 13-lipoxygenases under different stress conditions.     

Profiling DNA methylation by melting analysis

The idea of modifying DNA with bisulfite has paved the way for a variety of polymerase chain reaction (PCR) methods for accurately mapping 5-methylcytosine at specific genes. Bisulfite selectively deaminates cytosine to uracil under conditions where 5-methylcytosine remains unreacted. Following conventional PCR amplification of bisulfite-treated DNA, original cytosines appear as thymine while 5-methylcytosines appear as cytosine. Because the relative thermostability of a DNA duplex increases with increasing content of G:C base pairs, PCR products originating from DNA templates with different contents of 5-methylcytosine differ in melting temperature, i.e., the temperature required to convert the double helix into random coils. We describe two methods that resolve differentially methylated DNA sequences on the basis of differences in melting temperature. The first method integrates PCR amplification of bisulfite-treated DNA and subsequent melting analysis by using a thermal cycler coupled with a fluorometer. By including in the reaction a PCR-compatible, fluorescent dye that specifically binds to double-stranded DNA, the melting properties of the PCR product can be examined directly in the PCR tube by continuous fluorescence monitoring during a temperature transition. The second method relies on resolution of alleles with different 5-methylcytosine contents by analysis of PCR products in a polyacrylamide gel containing a gradient of chemical denaturants. Optimal resolution of differences in melting temperature is achieved by a special design of PCR primers. Both methods allow resolution of "heterogeneous" methylation, i.e., the situation where the content and distribution of 5-methylcytosine in a target gene differ between different molecules in the same sample.     

A chimeric Anabaena/ Escherichia coli KdpD protein (Anacoli KdpD) functionally interacts with E. coli KdpE and activates kdp expression in E. coli

The kdpFABC operon, coding for a high-affinity K(+)-translocating P-type ATPase, is expressed in Escherichia coli as a backup system during K(+) starvation or an increase in medium osmolality. Expression of the operon is regulated by the membrane-bound sensor kinase KdpD and the cytosolic response regulator KdpE. From a nitrogen-fixing cyanobacterium, Anabaena sp. strain L-31, a kdpDgene was cloned (GenBank accession no. AF213466) which codes for a KdpD protein (365 amino acids) that lacks both the transmembrane segments and C-terminal transmitter domain and thus is shorter than E. coli KdpD. A chimeric kdpD gene was constructed and expressed in E. coli coding for a protein (Anacoli KdpD), in which the first 365 amino acids of E. coli KdpD were replaced by those from Anabaena KdpD. In everted membrane vesicles, this chimeric Anacoli KdpD protein exhibited activities, such as autophosphorylation, transphosphorylation and ATP-dependent dephosphorylation of E. coli KdpE, which closely resemble those of the E. coli wild-type KdpD. Cells of E. coli synthesizing Anacoli KdpD expressed kdpFABC in response to K(+) limitation and osmotic upshock. The data demonstrate that Anabaena KdpD can interact with the E. coliKdpD C-terminal domain resulting in a protein that is functional in vitro as well as in vivo.     

Amino acid replacements in transmembrane domain 1 influence osmosensing but not K<Superscript>+</Superscript> sensing by the sensor kinase KdpD of <Emphasis Type="Italic">Escherichia coli</Emphasis>

Expression of the kdpFABC operon coding for the high affinity K+ -translocating KdpFABC complex of Escherichia coli is induced by K+ limitation or high osmolality. This process is controlled by the sensor kinase/response regulator system KdpD/KdpE. To study the importance of the transmembrane domains of KdpD for stimulus perception, each amino acid residue of the transmembrane domain 1 and Asp-424 of the adjacent periplasmic loop were replaced with Cys in a KdpD derivative devoid of native Cys residues. In vivo analysis of KdpD proteins with a single Cys residue revealed that 14 out of 18 amino acid replacements caused an altered response towards an osmotic upshift imposed by NaCl, whereby only four replacements also altered the response towards changes in the K+ concentration. The in vitro activities of most of the KdpD derivatives were in the range of KdpD devoid of native Cys residues. The results reveal that the osmosensing and K+ -sensing properties of KdpD can be dissected. Furthermore, the data support the hypothesis that osmosensing involves amino acid residues of the transmembrane domains.     

Probing the endocytic pathway in live cells using dual-color fluorescence cross-correlation analysis

Fluorescence (auto)correlation spectroscopy (FCS) has developed into a widely used method for investigating molecular dynamics and mobility of molecules in vitro and in vivo. Dual-color cross-correlation, an extension of this technique, also assesses the concomitant movement of two spectrally distinguishable fluorescent molecules and has therefore proven superior to autocorrelation analysis to study interactions between different molecular species in solution. Here we explore the benefits of cross-correlation analysis when applied to live cells, by demonstrating its potential in analyzing endocytic processes. Bacterial cholera toxin (CTX) was labeled with Cy2 and Cy5 dyes on different subunits of the same holotoxin. Along the endocytic pathway, positive cross-correlation between the A and B subunits was first preserved, later followed by a loss in cross-correlation upon their separation in the Golgi. Furthermore, endocytosis of a mixture of only Cy2- and only Cy5-labeled holotoxins also gave rise to cross-correlation. Our results suggest that cross-correlation may be used to recognize whether different cargoes use the same endocytic pathway. Additionally, we show that cross-correlation is applicable to two-dimensional membrane diffusion. CTX bound to GM1-containing artificial giant unilamellar vesicles was diffusible, whereas CTX bound to the plasma membrane was immobile on the FCS time-scale, possibly because of raft-association of GM1.