Cell Entry of C3 Exoenzyme from Clostridium botulinum

Abstract

Clostridium botulinum C3 is the prototype of C3-like ADP-ribosyltransferases that selectively ADP-ribosylate the small GTP-binding proteins RhoA/B/C and inhibit their downstream signaling pathways. It is used as pharmacological tool to study cellular Rho functions. In addition, C3bot harbors a transferase-independent activity on neurons to promote axonal and dendritic growth and branching. Many bacterial protein toxins interact specifically with proteins and/or other membrane components at the surface of target cells. Binding enables access to the appropriate cellular compartment so that the knowledge of the receptor allows essential insight into the mechanism of these toxins. Unlike other bacterial protein toxins (such as the clostridial C1 and C2 toxins from C. botulinum), C3 exoenzyme is devoid of a binding and translocation domain, with which toxins usually initiate receptor-mediated endocytosis followed by access to the intact cell. To date, no specific mechanism for internalization of C3 exoenzyme has been identified. Recently, vimentin was identified as membranous C3-binding partner involved in binding and uptake of C3. Although vimentin is not detected in neurons, vimentin is re-expressed after damage in regenerating neurons. Reappearance of vimentin allows C3 to get access to lesioned neurons/axons to exhibit axonotrophic and dentritotrophic effects.

An In-Silico Sequence-Structure-Function Analysis of the N-Terminal Lobe in CT Group Bacterial ADP-Ribosyltransferase Toxins

“…Indeed, Rohrbeck and colleagues [51] identified a membrane partner that binds C3bot1, and vimentin (rod domain) was established as the cellular receptor in neuronal and macrophage cell lines. A more recent study showed the 88- R GD-90 sequence in C3bot1 functions as a vimentin binding-motif in neuronal cells [52]. Effectively the R GD motif is unique in C3-etoxins; however, it should be noted that this Arg residue (at β 1 ) is an invariant residue in all CT-like toxins (the signature “R”) involved in the binding of the NAD + substrate.…”

Several New Putative Bacterial ADP-Ribosyltransferase Toxins Are Revealed from In Silico Data Mining, Including the Novel Toxin Vorin, Encoded by the Fire Blight Pathogen Erwinia amylovora

“…Translocation of CT-like toxin A-domain is accomplished in a similar manner to DT-like toxins, where a B-subunit pentamer forms the receptor-binding domain [ 24 , 25 ]. In contrast, C3-like toxins are A-only and enter host cells autonomously via a currently unknown mechanism [ 26 ].…”

Lysosomal swelling and lysis mediate delivery of C3 toxins to their cytoplasmic targets

“…ADP‐ribosylation by C3 toxins inhibits Rho signaling, resulting in the dysregulation of cytoskeletal actin filaments (Genth et al, 2003; Wheeler & Ridley, 2004). Compared to other Rho‐targeting enzymes, C3 toxins display high specificity for the RhoA, B, and C isoforms, which has promoted their routine use as inhibitors in cell biology studies (Rohrbeck & Just, 2017; Wheeler & Ridley, 2004; Wilde et al, 2000). Yet, despite their contribution to understanding Rho‐GTPases, little is known about the pathways utilized by C3 toxins to reach their target.…”