TETRAMERIZATION OF THE AKT1 PLANT POTASSIUM CHANNEL INVOLVES ITS C-TERMINAL CYTOPLASMIC DOMAIN

All plant channels identified so far show high conservation throughout the polypeptide sequence except in the ankyrin domain which is presen t only in those closely related to AKT1. In this study, the architectu re of the AKT1 protein has been investigated. AKT1 polypeptides expres sed in the baculovirus/Sf9 cells system were found to assemble into te tramers as observed with animal Shaker-like potassium channel subunits . The AKT1 C-terminal intracytoplasmic region (downstream from the tra nsmembrane domain) alone formed tetrameric structures when expressed i n Sf9 cells, revealing a tetramerization process different from that o f Shaker channels. Tests of subfragments from this sequence in the two -hybrid system detected two kinds of interaction. The first, involving two identical segments (amino acids 371-516), would form a contact be tween subunits, probably via their putative cyclic nucleotide-binding domains. The second interaction was found between the last 81 amino ac ids of the protein and a region lying between the channel hydrophobic core and the putative cyclic nucleotide-binding domain. As the interac ting regions are highly conserved in all known plant potassium channel s, the structural organization of AKT1 is likely to extend to these ch annels. The significance of this model with respect to animal cyclic n ucleotide-gated channels is also discussed.