Key resources table.

Reagent type (species) or resource	Designation	Source or reference	Identifiers	Additional information
Gene	Erwinia ligand-gated ion channel (ELIC)	Synthetic gene from Genscript	UniProt P0C7B7
Gene	Human 5-hydroxytryptamine receptor 3A (5-HT3A R)	John Peters laboratory (Belelli et al., 1995)	GenBank NM_000869
Cell line	C43 E. coli strain	Lucigen	#60446
Cell line	WK6 E. coli strain	Zell and Fritz, 1987
Cell line	TG1 E. coli strain	Lucigen	#60502
Antibody	PAM-Nb			Nanobody obtained by immunizing an adult llama glama with recombinant ELIC protein. PAM-Nb was used at 1–30 μM concentration in electrophysiology experiments. In protein crystals the PAM-Nb concentration is > 1 mM.
Antibody	NAM-Nb			Nanobody obtained by immunizing an adult llama glama with recombinant ELIC protein. NAM-Nb was used at 0.03–3 μM concentration in electrophysiology experiments. In protein crystals the NAM-Nb concentration is > 1 mM.
Recombinant DNA reagent	pGEM-HE	Liman et al. (1992)
Recombinant DNA reagent	pMESy4		GenBank KF415192
Sequence-based reagent	P111C oligonucleotide forward and reverse			5’-caccaagttgtccatcTGcacggacagcatctgg-3’ 5’-ccagatgctgtccgtgCAgatggacaacttggtg- 3’
Sequence-based reagent	T112C oligonucleotide forward and reverse			5’-caagttgtccatccccTGCgacagcatctgggtcc-3’ 5’-ggacccagatgctgtcGCAggggatggacaacttg-3’
Sequence-based reagent	N147C oligonucleotide forward and reverse			5’-caaggcgaagttcagTGctacaagccccttcagg-3’ 5’-cctgaaggggcttgtagCActgaacttcgccttg-3’
Sequence-based reagent	K149C oligonucleotide forward and reverse			5’-ggcgaagttcagaactacTGCccccttcaggtggtga-3’ 5’- tcaccacctgaaggggGCAgtagttctgaacttcgcc-3’
Sequence-based reagent	L151C oligonucleotide forward and reverse			5’- gttcagaactacaagcccTGtcaggtggtgactgc-3’ 5’-gcagtcaccacctgaCAgggcttgtagttctgaac-3’
Sequence-based reagent	Y86C oligonucleotide forward and reverse			5’- ctggtaccggcagtGctggactgatgag-3’ 5’-ctcatcagtccagCactgccggtaccag-3’
Sequence-based reagent	F125C oligonucleotide forward and reverse			5’-ggacattctcatcaatgagtGcgtggatgtggg-3’ 5’-cccacatccacgCactcattgatgagaatgtcc-3’
Sequence-based reagent	Primer for nanobody library generation CALL001	Pardon et al. (2014)		5′-GTCCTGGCTGCTCTTCTACAAGG-3′
Sequence-based reagent	Primer for nanobody library generation CALL002	Pardon et al. (2014)		5′-GGT ACGTGCTGTTGAACTGTTCC-3′
Sequence-based reagent	Primer for nanobody library generation VHH-Back	Pardon et al. (2014)		5′-GATGTGCAGCTGCAG GAGTCTGGRGGAGG-3′(PstI)
Sequence-based reagent	Primer for nanobody library generation VHH-For	Pardon et al. (2014)		5′-CTAGTGCGGCCGCTGG AGACGGTGACCTGGGT-3′(Eco91I)
Sequence-based reagent	Primer for nanobody library analysis MP57	Pardon et al. (2014)		5′-TTATGCTTCCGGCTC GTATG-3′
Peptide, recombinant protein	Primer for nanobody library analysis GIII	Pardon et al. (2014)		5′-CCACAGACAGCCCTCATAG-3′
Commercial assay or kit	mMessage mMachine T7 Transcription kit	ThermoFisher Scientific	#AM1344
Commercial assay or kit	Ni Sepharose 6 Fast Flow resin	GE Healthcare	#17531802
Commercial assay or kit	Superdex 75 10/300 GL column	GE Healthcare	# 17-5174-01
Commercial assay or kit	Superdex 200 Increase 10/300 GL	GE Healthcare	# 28990944
Commercial assay or kit	QuikChange Site-Directed Mutagenesis Kit	Agilent	# 200518
Chemical compound, drug	5-HT creatinine sulphate	Sigma-Aldrich	#S2805
Chemical compound, drug	MTSEA-biotin	Biotium	#90064
Chemical compound, drug	GABA	Sigma-Aldrich	#A2129
Chemical compound, drug	E. coli total lipid extract	Avanti Polar Lipids	#100500P
Chemical compound, drug	Anagrade n-undecyl-β-D-maltoside (UDM)	Anatrace	#U300
Software, algorithm	GraphPad Prism Software v4.03		RRID:SCR_002798
Software, algorithm	CCP4	Winn et al., 2011
Software, algorithm	STARANISO	Tickle et al., 2018 http://staraniso.globalphasing.org
Software, algorithm	XDS	Kabsch, 2010
Software, algorithm	Coot	Emsley et al. (2010)
Software, algorithm	Buster	Smart et al. (2012)
Software, algorithm	PDB-REDO	Joosten et al. (2014)
Software, algorithm	MolProbity	Chen et al. (2010)
Software, algorithm	PyMOL v2.3.0	Schrödinger	RRID:SCR_000305
Software, algorithm	HOLE	Smart et al. (1996)
Software, algorithm	CAVER	Jurcik et al. (2018)
Software, algorithm	PHENIX	Adams et al. (2010)
Software, algorithm	HiClamp	Multi Channel Systems