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1.A.9.9.3
X-ray structures of CLIC of 680 aas and 1N-terminal and 4 C-terminal TMSs with a large N-terminal hydrophilic domain (NTD) from a Desulfofustis deltaproteobacterium have been solved. The protein includes a periplasmic NTD fused to the conventional ligand-binding domain (LBD) (Hu et al. 2020). The NTD consists of two jelly-roll domains interacting across each subunit interface. Binding of Ca2+ at the LBD subunit interface is associated with a closed transmembrane pore, with resolved monovalent cations intracellular to the hydrophobic gate. Accordingly, DeCLIC-injected oocytes conducted currents only upon depletion of extracellular Ca2+. DeCLIC crystallized in the absence of Ca2+ with a wide-open pore and remodeled periplasmic domains, including increased contacts between the NTD and classic LBD agonist-binding sites. Functional, structural, and dynamical properties of DeCLIC paralleled those of sTeLIC, a pLGIC from another symbiotic prokaryote. Based on these DeCLIC structures, the previous structure of bacterial ELIC (the first high-resolution structure of a pLGIC) should be reclassified as a "locally closed" conformation. Structures of DeCLIC in multiple conformations illustrate dramatic conformational state transitions and diverse regulatory mechanisms available to ion channels in pLGICs, particularly involving Ca2+ modulation and periplasmic NTDs (Hu et al. 2020). Ligand-binding affinities are insensitive to binding-site occupancy, and mutations in residues in the transmembrane domain are unlikely to affect the channel's affinities for ligands that bind to the extracellular domain (Godellas and Grosman 2022). Cation-selective pLGICs contain a long helical extension (MA) of one of the transmembrane helices. The MA helix affects both the membrane expression of, and ion conductance levels through, these pLGICs. In fact, the MA helix Is important for receptor assembly and function in the alpha4beta2 nACh receptor (Fricska et al. 2023).

Accession Number:ESQ11000.1
Protein Name:ESQ11000.1 hypothetical protein N839_03575 [uncultured Desulfofustis sp. PB-SRB1]
Length:643
Molecular Weight:
Species:uncultured Desulfofustis sp. PB-SRB1 [1385624]
Number of TMSs:5
Substrate ion, monoatomic monocation

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FASTA formatted sequence
1:	MHNLQQLLPT RSLIWIFSFL TSISIWCTVA HAETEGRVQH FTGYIEDGRG IFYSLPDMKQ 
61:	GDIIYASMQN TGGNLDPLVG IMAEEIDPAV SLGQVLEKAL ASENDLISEL TAVADRIFLG 
121:	WDDDGGKGYS ASLEFTIPRD GTYHIFAGST ITNQRLDKFQ PTYTTGSFQL ILGLNAPQVI 
181:	SGEGEPEGEV FASLASLEIK PEAHVQELEI RLDKDTRYLT QHTRNLQPGD TFHALVEPIG 
241:	EAPLPRLRLT DSGGKPLAFG LIDQPGESVE LNYTCDQDIC ELVVHVDGTD GQKDSGEAVY 
301:	RLLVGINAPN LRESGQTPVG SSVFLESDLV TVGLAVDQIV GVDQRSENFS VVGTLKLSWH 
361:	DPKLGFSPDQ CGCTVKSFED ASIRAVAGEI NLPLPSFSFY NQQGNRWSQN QVIFVTPDGR 
421:	ASYFERFTVT LQAPDFDFLA YPFDRQKFSI KVDLAVPTNM FIFNEIERFQ QVVGDQLGEE 
481:	EWVVTSYSQE ITEVPFERGS TNSRFTTTLL VKRNLEYYIL RIFVPLFLII SVSWVIFFLK 
541:	DYGRQLEVAS GNLLVFVAFN FTISGDLPRL GYLTVLDRFM IVSFCLTAIV VLISVCQKRL 
601:	GAVGKQAVAA QIDTWVLVIY PLVYSLYIIW VYLRFFTDHI GW