On diphtheria toxin fragment A release into the cytosol-Cytochalasin D effect and involvement of actin filaments and eukaryotic elongation factor 2
Istanbul University, Istanbul Faculty of Medicine, Department of Biophysics, 34390 Çapa, Istanbul, Türkiye.The international journal of biochemistry & cell biology (Impact Factor: 4.05). 06/2011; 43(9):1365-72. DOI: 10.1016/j.biocel.2011.05.017
Diphtheria toxin has been well characterized in terms of its receptor binding and receptor mediated endocytosis. However, the precise mechanism of the cytosolic release of diphtheria toxin fragment A from early endosomes is still unclear. Various reports differ regarding the requirement for cytosolic factors in this process. Here, we present data indicating that the distribution of actin filaments due to cytochalasin D action enhances the retention of diphtheria toxin in early endosomes. Treating cells with cytochalasin D reduces the cytosolic fragment A activity and leads to changes in the intracellular distribution and size of early endosomes with toxin cargo. F-actin and eukaryotic elongation factor 2 can promote fragment A release from toxin-loaded early endosomes in an in vitro translocation system. Moreover, these proteins bind to toxin-loaded early endosomes in vitro and promote each other's binding. They are thus thought to be involved in the cytosolic release of fragment A. Finally, ADP-ribosylation of eukaryotic elongation factor 2 is shown to inhibit fragment A release and, via a feed-back mechanism, to account for the minute amounts of fragment A normally found in the cytosol.
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ABSTRACT: Diphtheria toxin (DT) and its N-terminal fragment A (FA) catalyse the transfer of the ADP-ribose moiety of nicotinamide adenine dinucleotide (NAD) into a covalent linkage with eukaryotic elongation factor 2 (eEF2). DT-induced cytotoxicity is versatile, and it includes DNA cleavage and the depolymerisation of actin filaments. The inhibition of the ADP-ribosyltransferase (ADPrT) activity of FA did not affect the deoxyribonuclease activity of FA or its interaction with actin. The toxin entry rate into cells (HUVEC) was determined by measuring the ADP-ribosyltransferase activity. DT uptake was nearly 80% after 30 min. The efficiency was determined as K(m) = 2.2 nM; V(max) = 0.25 pmol.min(-1). The nuclease activity was tested with hyperchromicity experiments, and it was concluded that G-actin has an inhibitory effect on DT nuclease activity. In the presence of DT and mutant of diphtheria toxin (CRM197), F-actin depolymerisation was determined with gel filtration, WB and fluorescence techniques. In the presence of DT and CRM197, 60-65% F-actin depolymerisation was observed. An in vitro FA-actin interaction and F-actin depolymerisation were reported in our previous paper. The present study thus confirms the depolymerisation of actin cytoskeleton in vivo.
Chapter: Toxin Structure, Delivery and Action[Show abstract] [Hide abstract]
ABSTRACT: Diphtheria toxin (DTx) consists of a 535 amino acids polypeptide and contains the following three domains: the amino terminal fragment A (FA or catalytic C-domain) that catalyses the transfer of an ADP-ribosyl group of NAD+ to a post-translationally modified histidine (diphthamide) residue on eukaryotic elongation factor 2 (eEF2) and inhibits protein synthesis. Fragment B (FB) consist of the carboxy terminal receptor-binding R-domain, and the translocation (or transmembrane) T-domain. Following binding to its cell surface receptor via R-domain, DTx is internalized through the clathrin-dependent endocytosis. The acid pH created in the early endosomes triggers a conformational change in the toxin leading to the insertion of the T and C-domains in the membrane. The catalytic domain is then translocated into the cytosol across the early endosomal membrane and protein synthesis inhibition occurs. DTx-induced cytotoxicity is versatile, and it includes DNA cleavage and the depolymerisation of actin filaments. FA can interact with both G and F-actin. The binding to the latter appears to take place at the plus end of the filament blocking further polymerisation and it was concluded that G-actin has an inhibitory effect on DTx nuclease activity.
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