1.A.48 The Anion Channel Tweety (Tweety) Family
The Tweety family of anion channel proteins is present in animals and plants. These proteins contain about 500 - 1200 aas and 5 TMSs in an arrangement: 2 + 2 + 1, with an extra N-terminal TMS present in some plant homologues (e.g., NP_178935). They produce large conductance chloride (maxi-Cl-) currents. Calcium-activated chloride currents can be recorded in almost all cells. Tweety, a gene located in the Drosophila flightless locus, possesses five or six TMSs, and a human homologue of tweety (hTTYH3) is a large-conductance Ca2+-activated Cl- channel, while the related gene, hTTYH1, is a swelling-activated Cl- current. hTTYH3 is expressed in excitable tissues, including the heart, brain and skeletal muscle, whereas hTTYH1 is expressed mainly in the brain (Suzuki 2006). The hTTYH3-induced Cl- current had a linear current-voltage relationship, a large single-channel conductance (260 pS) and the anion permeability sequence I- > Br- > Cl-. The hTTYH3 channel shows complex gating kinetics and voltage-dependent inactivation, dependent on a micromolar intracellular Ca2+ concentration.
'Tweety has three human homologues (hTTYH1-3) which are all maxi-Cl- channel proteins (Suzuki and Mizuno, 2004). He et al., (2008) provided evidence for a structure for Tweety family proteins which incorporates five membrane-spanning domains with a topology in which the N-terminus is located extracellularly and the C-terminus cytoplasmically. N-glycosylation is important, but not essential, in the processing of members of the Tweety family. Incomplete N-glycosylation gives rise to reduced expression and increased ubiquitination (He et al., 2008).
hTTYH3 mRNA is distributed in excitable tissues. Positively charged amino acyl residues in the putative pore contribute to anion selectivity. The hTTYH3 single channel shows 26 picosiemen linear current voltage, complex kinetics and sensitivity to 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid. This channel and hTTYH2 encode ionomycin-induced maxi-Cl- channels, but hTTYH1 encodes a Ca2+-independent, swelling-activated maxi-Cl- channel (Suzuki and Mizuno, 2004).
Tweety homologs (TTYHs) are abundant in the brain. The three human paralogs were assigned to function as anion channels thatare either activated by Ca2+ or cell swelling. To uncover their unknown architecture and its relationship to function, Sukalskaia et al. 2021 determined the structures of human TTYH1-3 by cryo-EM. All structures display equivalent features of a dimeric membrane protein that contains five TMSs and an extended extracellular domain. As none of the proteins shows attributes reminiscent of an anion channel, the authors revisited functional experiments and did not find an indication of ion conduction. Instead, they found density in an extended hydrophobic pocket contained in the extracellular domain that emerges from the lipid bilayer, which suggested to them a role of TTYH proteins in interactions with lipid-like compounds residing in the membrane (Sukalskaia et al. 2021).
Homologues of Tweety are found in vertebrates, insects, worms and probably plants. They are prevalent in animals but scarce in plants. A search is in progress to identify homologues elsewhere.