ID   3SXB_MICFL              Reviewed;          84 AA.
AC   U3EPL2;
DT   11-DEC-2019, integrated into UniProtKB/Swiss-Prot.
DT   13-NOV-2013, sequence version 1.
DT   03-AUG-2022, entry version 24.
DE   RecName: Full=Three-finger toxin 3b {ECO:0000312|EMBL:JAB52853.1};
DE            Short=3FTx 3b {ECO:0000305};
DE   AltName: Full=Micrurus fulvius dimeric neurotoxin b {ECO:0000250|UniProtKB:U3FAE1};
DE   AltName: Full=SIGMA-neurotoxin {ECO:0000250|UniProtKB:U3FAE1};
DE   Flags: Precursor;
OS   Micrurus fulvius (Eastern coral snake) (Coluber fulvius).
OC   Eukaryota; Metazoa; Chordata; Craniata; Vertebrata; Euteleostomi;
OC   Lepidosauria; Squamata; Bifurcata; Unidentata; Episquamata; Toxicofera;
OC   Serpentes; Colubroidea; Elapidae; Elapinae; Micrurus.
OX   NCBI_TaxID=8637;
RN   [1]
RP   NUCLEOTIDE SEQUENCE [MRNA].
RC   TISSUE=Venom gland;
RX   PubMed=23915248; DOI=10.1186/1471-2164-14-531;
RA   Margres M.J., Aronow K., Loyacano J., Rokyta D.R.;
RT   "The venom-gland transcriptome of the eastern coral snake (Micrurus
RT   fulvius) reveals high venom complexity in the intragenomic evolution of
RT   venoms.";
RL   BMC Genomics 14:531-531(2013).
CC   -!- FUNCTION: Postsynaptic neurotoxin that produces potent, and completely
CC       reversible, postsynaptic neuromuscular blockade, as well as broad
CC       spectrum inhibition of human muscle and neuronal nicotinic
CC       acetylcholine receptors (nAChRs). Inhibition is potent or moderate,
CC       depending on the receptor (alpha-1-beta-1-delta-epsilon/CHRNA1-CHRNB1-
CC       CHRND-CHRNE (IC(50)=2.56 uM), alpha-4-beta-2/CHRNA4-CHRNB2 (IC(50)=1.8
CC       uM), alpha-7/CHRNA7 (IC(50)=7 uM), and alpha-3-beta-2/CHRNA3-CHRNB2
CC       (IC(50)=12.6 uM)). Acts as a competitive antagonist of ACh. Binds to
CC       chicken muscle-type nicotinic acetylcholine receptor (AChR) with high
CC       potency compared with the cloned human receptor. Unlike short-chain
CC       alpha-3FTxs that only bind to muscle nAChRs, this toxin utilizes
CC       dimerization to expand its pharmacological targets to block neuronal
CC       nAChRs. {ECO:0000250|UniProtKB:U3FAE1}.
CC   -!- SUBUNIT: Homodimer; non-covalently linked. Is able to form a tetramer
CC       of dimers in the presence of 2 zinc ions.
CC       {ECO:0000250|UniProtKB:U3FAE1}.
CC   -!- SUBCELLULAR LOCATION: Secreted {ECO:0000250|UniProtKB:U3FAE1}.
CC   -!- TISSUE SPECIFICITY: Expressed by the venom gland.
CC       {ECO:0000250|UniProtKB:U3FAE1}.
CC   -!- MISCELLANEOUS: Weakly inhibits human alpha-4-beta-4/CHRNA4-CHRNB4 and
CC       has no activity on human alpha-9-alpha-10/CHRNA9-CHRNA10 and human
CC       alpha-3-beta-4/CHRNA3-CHRNB4 nAChRs. {ECO:0000250|UniProtKB:U3FAE1}.
CC   -!- SIMILARITY: Belongs to the snake three-finger toxin family. Short-chain
CC       subfamily. {ECO:0000305}.
CC   ---------------------------------------------------------------------------
CC   Copyrighted by the UniProt Consortium, see https://www.uniprot.org/terms
CC   Distributed under the Creative Commons Attribution (CC BY 4.0) License
CC   ---------------------------------------------------------------------------
DR   EMBL; GAEP01001968; JAB52853.1; -; mRNA.
DR   AlphaFoldDB; U3EPL2; -.
DR   SMR; U3EPL2; -.
DR   GO; GO:0005576; C:extracellular region; IEA:UniProtKB-SubCell.
DR   GO; GO:0030550; F:acetylcholine receptor inhibitor activity; IEA:UniProtKB-KW.
DR   GO; GO:0046872; F:metal ion binding; IEA:UniProtKB-KW.
DR   GO; GO:0090729; F:toxin activity; IEA:UniProtKB-KW.
DR   Gene3D; 2.10.60.10; -; 1.
DR   InterPro; IPR045860; Snake_toxin-like_sf.
DR   SUPFAM; SSF57302; SSF57302; 1.
PE   3: Inferred from homology;
KW   Acetylcholine receptor inhibiting toxin; Disulfide bond; Metal-binding;
KW   Neurotoxin; Postsynaptic neurotoxin; Secreted; Signal; Toxin; Zinc.
FT   SIGNAL          1..21
FT                   /evidence="ECO:0000255"
FT   CHAIN           22..84
FT                   /note="Three-finger toxin 3b"
FT                   /evidence="ECO:0000250|UniProtKB:U3FAE1"
FT                   /id="PRO_5004640414"
FT   BINDING         50
FT                   /ligand="Zn(2+)"
FT                   /ligand_id="ChEBI:CHEBI:29105"
FT                   /evidence="ECO:0000250|UniProtKB:U3FAE1"
FT   SITE            58
FT                   /note="Contributes to dimerization"
FT                   /evidence="ECO:0000250|UniProtKB:U3FAE1"
FT   SITE            61
FT                   /note="Contributes to dimerization"
FT                   /evidence="ECO:0000250|UniProtKB:U3FAE1"
FT   SITE            64
FT                   /note="Contributes to dimerization"
FT                   /evidence="ECO:0000250|UniProtKB:U3FAE1"
FT   DISULFID        24..41
FT                   /evidence="ECO:0000250|UniProtKB:U3FAE1"
FT   DISULFID        34..59
FT                   /evidence="ECO:0000250|UniProtKB:U3FAE1"
FT   DISULFID        63..71
FT                   /evidence="ECO:0000250|UniProtKB:U3FAE1"
FT   DISULFID        72..77
FT                   /evidence="ECO:0000250|UniProtKB:U3FAE1"
SQ   SEQUENCE   84 AA;  9155 MW;  A65C29F1543952A9 CRC64;
     MKTLLLTLVV VTIVCLDLGN SLKCYSSRTE TMTCPEGEDK CEKYAAGLMH GSFFFIYTCT
     SKCHEGAYNV CCSTDLCNKS STSG