Function
Botulinum neurotoxin A heavy chain: Responsible for host epithelial cell transcytosis, host nerve cell targeting and translocation of botulinum neurotoxin A light chain (LC) into host cytosol. Composed of 3 subdomains; the translocation domain (TD), and N-terminus and C-terminus of the receptor-binding domain (RBD) (PubMed:19096517). The RBD is responsible for binding to host epithelial cells and transcytosis across them; this uses different receptors than those on nerve cells (PubMed:21106906). RBD is also responsible for adherence of toxin to host nerve cell surface; HC alone prevents uptake of whole toxin by neural cells, and delays paralysis onset by 75% (PubMed:6694738, PubMed:10413679). Isolated RBD also delays paralysis onset (PubMed:21106906). The N-terminus of the RBD binds to phosphatidylinositol, which might play a role in membrane-binding (PubMed:19161982). Binds to host protein receptor synaptic vesicle glycoproteins SV2A, SV2B and SV2C via lumenal loop 4 (PubMed:16545378, PubMed:6370252, PubMed:27294781, PubMed:24240280, PubMed:19650874, PubMed:27313224). Binding can be inhibited by protein fragments from either the HC or SV2C (PubMed:24240280). Isolated HC significantly decreases uptake and toxicity of whole BoNT/A, but also interferes with uptake of BoNT/E and to a lesser extent BoNT/F (PubMed:19650874). The RBD recognizes the N-linked glycan on 'Asn-559' of SV2A, SV2B and SV2C; hydrogen-bonding occurs via 10 well-defined water molecules and stacking of hydrophobic residues (PubMed:27294781). Binds one host GT1b ganglioside, which serves as a coreceptor (PubMed:14731268, PubMed:18704164, PubMed:27958736). Modeling shows the HC can bind both coreceptors (a ganglioside and SV2 protein) simultaneously at different sites (PubMed:24240280). Crystals of the RBD with a GT1b analog can be grown at pH 5.5, indicating the toxin-ganglioside complex could be stable within the endosome (PubMed:18704164). Isolated RBD binds NTNHA (a bacterial protein that protects toxin) with high affinity at pH 6.0 but not at pH 7.5 (PubMed:22363010). The N-terminal belt (residues 449-545) wraps around the perimeter of the LC, probably protecting Zn(2+) in the active site; it is not required for channel formation by the TD domain but may serve to prevent premature LC dissociation from the translocation channel and to protect toxin prior to translocation (PubMed:22158863, PubMed:17907800, PubMed:19351593). The isolated TD forms transmembrane channels of about 15 Angstroms in the absence of a pH gradient; LC translocation requires a pH and redox gradient (pH 5.0/oxidizing in the cis compartment, pH 7.0/reducing in the trans compartment), LC does not unfold unless the cis pH is 6.0 or less (PubMed:2446925, PubMed:17666397, PubMed:19096517). Pores are presumably made by 1-2 toxin molecules (PubMed:23471747). While interaction with the RBD modulates the pH threshold for membrane insertion, the RBD is not essential for toxin degradation of SNAP25 in neural cells (PubMed:19096517).
Sequence
MPFVNKQFNYKDPVNGVDIAYIKIPNVGQMQPVKAFKIHNKIWVIPERDTFTNPEEGDLNPPPEAKQVPVSYYDSTYLSTDNEKDNYLKGVTKLFERIYSTDLGRMLLTSIVRGIPFWGGSTIDTELKVIDTNCINVIQPDGSYRSEELNLVIIGPSADIIQFECKSFGHEVLNLTRNGYGSTQYIRFSPDFTFGFEESLEVDTNPLLGAGKFATDPAVTLAHELIHAGHRLYGIAINPNRVFKVNTNAYYEMSGLEVSFEELRTFGGHDAKFIDSLQENEFRLYYYNKFKDIASTLNKAKSIVGTTASLQYMKNVFKEKYLLSEDTSGKFSVDKLKFDKLYKMLTEIYTEDNFVKFFKVLNRKTYLNFDKAVFKINIVPKVNYTIYDGFNLRNTNLAANFNGQNTEINNMNFTKLKNFTGLFEFYKLLCVRGIITSKTKSLDKGYNKALNDLCIKVNNWDLFFSPSEDNFTNDLNKGEEITSDTNIEAAEENISLDLIQQYYLTFNFDNEPENISIENLSSDIIGQLELMPNIERFPNGKKYELDKYTMFHYLRAQEFEHGKSRIALTNSVNEALLNPSRVYTFFSSDYVKKVNKATEAAMFLGWVEQLVYDFTDETSEVSTTDKIADITIIIPYIGPALNIGNMLYKDDFVGALIFSGAVILLEFIPEIAIPVLGTFALVSYIANKVLTVQTIDNALSKRNEKWDEVYKYIVTNWLAKVNTQIDLIRKKMKEALENQAEATKAIINYQYNQYTEEEKNNINFNIDDLSSKLNESINKAMININKFLNQCSVSYLMNSMIPYGVKRLEDFDASLKDALLKYIYDNRGTLIGQVDRLKDKVNNTLSTDIPFQLSKYVDNQRLLSTFTEYIKNIINTSILNLRYESNHLIDLSRYASKINIGSKVNFDPIDKNQIQLFNLESSKIEVILKNAIVYNSMYENFSTSFWIRIPKYFNSISLNNEYTIINCMENNSGWKVSLNYGEIIWTLQDTQEIKQRVVFKYSQMINISDYINRWIFVTITNNRLNNSKIYINGRLIDQKPISNLGNIHASNNIMFKLDGCRDTHRYIWIKYFNLFDKELNEKEIKDLYDNQSNSGILKDFWGDYLQYDKPYYMLNLYDPNKYVDVNNVGIRGYMYLKGPRGSVMTTNIYLNSSLYRGTKFIIKKYASGNKDNIVRNNDRVYINVVVKNKEYRLATNASQAGVEKILSALEIPDVGNLSQVVVMKSKNDQGITNKCKMNLQDNNGNDIGFIGFHQFNNIAKLVASNWYNRQIERSSRTLGCSWEFIPVDDGWGERPL