The N terminus and transmembrane segment S1 of Kv1.5 can coassemble with the rest of the channel independently of the S1–S2 linkage

The voltage-gated potassium channel Kv1.5 belongs to the Shaker superfamily. Kv1.5 is composed of four subunits, each comprising 613 amino acids, which make up the N terminus, six transmembrane segments (S1–S6), and the C terminus. We recently demonstrated that, in HEK cells, extracellularly applied proteinase K (PK) cleaves Kv1.5 channels at a single site in the S1–S2 linker. This cleavage separates Kv1.5 into an N-fragment (N terminus to S1) and a C-fragment (S2 to C terminus). Interestingly, the cleavage does not impair channel function. Here, we investigated the role of the N terminus and S1 in Kv1.5 expression and function by creating plasmids encoding various fragments, including those that mimic PK-cleaved products. Our results disclosed that although expression of the pore-containing fragment (Frag(304–613)) alone could not produce current, coexpression with Frag(1–303) generated a functional channel. Immunofluorescence and biotinylation analyses uncovered that Frag(1–303) was required for Frag(304–613) to traffic to the plasma membrane. Biochemical analysis revealed that the two fragments interacted throughout channel trafficking and maturation. In Frag(1–303)+(304–613)-coassembled channels, which lack a covalent linkage between S1 and S2, amino acid residues 1–209 were important for association with Frag(304–613), and residues 210–303 were necessary for mediating trafficking of coassembled channels to the plasma membrane. We conclude that the N terminus and S1 of Kv1.5 can attract and coassemble with the rest of the channel (i.e. Frag(304–613)) to form a functional channel independently of the S1–S2 linkage.