AKv1.1a is an Aplysia Kv1 channel close to a mammalian Kv1.4. Both channels show a prominent frequency-dependent cumulative inactivation. The cumulative inactivation of AKv1.1a but not of rKv1.4 was enhanced by the patch excision. To gain structural information about the phenomenon, we examined chimeras of AKv1.1a and rKv1.4. Chimeras with the AKv1.1a pore domain displayed enhanced cumulative inactivation after patch excision. In the pore domain, eight amino acids are different between the two channels. We, therefore, constructed eight mutants of AKv1.1a (A378E, D379P, Q380T, K384Q, R406K, G409T, W411G, L414I) based on the sequence differences. All the mutants showed a similar macroscopic current decay, suggesting that N-type inactivation was not affected. The patch excision failed to enhance the cumulative inactivation in A378E, D379P, G409T, and L414I. P/C-type inactivation in N-terminal deletion mutants (Delta N) became slower in A378E, D379P, G409T and L414I. Internal application of the N-terminal peptide from the ShB channel induces a frequency-dependent block of Delta N-AKv1.1a. By contrast, the block was not frequency dependent in Delta N-A378E, Delta N-D379P and Delta N-G409T. The results suggest that the positions 378, 379, 409, and perhaps 414 in the AKv1.1a backbone are involved in the stability of P/C-type inactivation and the cumulative inactivation of Kv1 channels.

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Structurally tachykinin-related peptides have been isolated from various invertebrate species and shown to exhibit their biological activities through a G-protein-coupled receptor (GPCR) for a tachykinin-related peptide. In this paper, we report the identification of a novel tachykinin-related peptide receptor, the urechistachykinin receptor (UTKR) from the echiuroid worm, Urechis unitinctus. The deduced UTKR precursor includes seven transmembrane domains and typical sites for mammalian tachykinin receptors and invertebrate tachykinin-related peptide receptors. A functional analysis of the UTKR expressed in Xenopus oocytes demonstrated that UTKR, like tachykinin receptors and tachykinin-related peptide receptors, activates calciumdependent signal transduction upon binding to its endogenous ligands, urechistachykinins (Uru-TKs) IÀV and VII, which were isolated as Urechis tachykinin-related peptides from the nervous tissue of the Urechis unitinctus in our previous study. UTKR responded to all Uru-TKs equivalently, showing that UTKR possesses no selective affinity with Uru-TKs. In contrast, UTKR was not activated by substance P or an Uru-TK analog containing a C-terminal Met-NH 2 instead of Arg-NH 2 . Furthermore, the genomic analysis revealed that the UTKR gene, like mammalian tachykinin receptor genes, consists of five exons interrupted by four introns, and all the intron-inserted positions are completely compatible with those of mammalian tachykinin receptor genes. These results suggest that mammalian tachykinin receptors and invertebrate tachykinin-related peptide receptors were evolved from a common ancestral GPCR gene. This is the first identification of an invertebrate tachykinin-related peptide receptor from other species than insects and also of the genomic structure of a tachykininrelated peptide receptor gene.

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