Fig. 2. Alignment of the seven predicted transmembrane helices of the two H. trivolvis receptors (Hel1, Hel2) with representative vertebrate (Homo sapiens; Hum1D, Hum7) and invertebrate (Drosophila melanogaster; Dmel1, Dmel7) 5-HT₁ and 5-HT₇ receptors. The conserved residues that have been shown to be involved in 5-HT binding to the receptor are marked with asterisks. The arrows along transmembrane (TM) helices 3 to 7 indicate the portion of the sequence that forms the vestibule leading from the extracellular space to the 5-HT binding site.