Variations in the Botulinum Neurotoxin Binding Domain and the Potential for Novel Therapeutics

Abstract

Botulinum neurotoxins (BoNTs) are categorised into immunologically distinct serotypes BoNT/A to /G). Each serotype can also be further divided into subtypes based on differences in amino acid sequence. BoNTs are ~150 kDa proteins comprised of three major functional domains: an N-terminal zinc metalloprotease light chain (LC), a translocation domain (H_N), and a binding domain (H_C). The H_C is responsible for targeting the BoNT to the neuronal cell membrane, and each serotype has evolved to bind via different mechanisms to different target receptors. Most structural characterisations to date have focussed on the first identified subtype within each serotype (e.g., BoNT/A1). Subtype differences within BoNT serotypes can affect intoxication, displaying different botulism symptoms in vivo, and less emphasis has been placed on investigating these variants. This review outlines the receptors for each BoNT serotype and describes the basis for the highly specific targeting of neuronal cell membranes. Understanding receptor binding is of vital importance, not only for the generation of novel therapeutics but also for understanding how best to protect from intoxication.

Keywords: SV2; binding domain; botulinum neurotoxins; ganglioside; neurones; synaptotagmin.

Figure 1

(a) Schematic of botulinum neurotoxin (BoNT) domain organisation. The crystal structures of (b) BoNT/A [10] and (c) BoNT/E [11] show different orientations of the receptor-binding domain (H_C) with respect to the rest of the molecule. H_N: translocation domain; HC: heavy chain; LC: light chain.

Figure 2

Mechanism of BoNT intoxication. (a) BoNT binds the neuronal cell membrane through a dual-receptor complex. (b) The BoNT–receptor complex is endocytosed and enclosed within a vesicle. (c) Acidification of the endocytic vesicle causes a conformational change, allowing the LC to be translocated through the membrane. (d) Thioredoxin (Trx), bound to the vesicle membrane, catalyses the reduction of a disulphide bond that releases the LC into the cytoplasm where it can cleave its soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) substrate. Cleavage of any one of the SNARE proteins of the SNARE complex inhibits membrane fusion and acetylcholine (ACh) release, thus stopping muscle contraction. ACh: acetylcholine; AChE: acetylcholine esterase; Trx: thioredoxin; TrxR: thioredoxin reductase.