Human ether-a-go-go-related gene (HERG) channels heterologously expressed in Xenopus oocytes are regulated by the activation of G protein-coupled hormone receptors that, like the thyrotropin-releasing hormone (TRH) receptor, activate phospholipase C. Previous work with serially deleted HERG mutants suggested that residues 326-345 located in the proximal domain of the channels amino terminus might be required for the hormonal modulation of HERG activation. Generation of new channel mutants deleted in this region further point to the amino acid sequence between residues 326 and 332 as a possible determinant of the TRH effects, but individual or combined single-point mutations in this sequence demonstrate that maintenance of its consensus sites for phosphorylation and/or interaction with regulatory components is not important for the modulatory response(s). The TRH-induced effects also remained unaltered when a basic amino acid cluster located between residues 362 and 366 is eliminated. Additionally, no effect of TRH was observed in channels carrying single-point mutations at the beginning of the intracellular loop linking transmembrane domains S4 and S5. Our results indicate that a correct structural arrangement of the amino terminal domains is essential for the hormone-induced modifications of HERG activation. They also suggest that the hormonal regulatory action is transmitted to the transmembrane channel core through interactions between the cytoplasmic domains and the initial portion of the S4-S5 linker.