Both N- and C-terminal regions contribute to the assembly and functional expression of homo- and heteromultimeric voltage-gated K+ channels

The functional diversity of voltage-gated K+ channels may be partially determined by the mechanisms that permit or limit the assembly of molecularly diverse K+ channel subunits. To determine possible amino acid sequence domains required for subunit assembly and expression, we have constructed 15 N- and C-terminal interstitial or truncation deletion mutations in mKv1.1 (MBK1), a mouse Shaker-like K+ channel. We injected Xenopus oocytes with cRNA encoding each of these mutants and coinjected each mutant cRNA with cRNA for wild-type mKv1.3, another mouse Shaker-like K+ channel that can form heteromultimers with mKv1.1. We found that the last five amino acids of the C-terminus of mKv1.1 contribute to functional expression by (1) rescuing the function of mutants with a large truncation of the C-terminus (delta 424-495), and (2) contributing to the slow inactivation kinetics (time constant of 2-3 sec) of wild-type mKv1.1 whole-cell K+ currents. All C-terminal deletion mutants were able to express at least as heteromultimers with mKv1.3, suggesting that the C-terminus is not required for channel assembly. In contrast, nine different interstitial or truncation mutants in which part of a highly conserved, large (80-99 amino acid residues) domain within the N-terminus had been deleted were unable to express either homomultimers or heteromultimers. The relatively small sizes and nonoverlapping distributions of the interstitial deletions enable us to suggest that the structural integrity of this entire N-terminal domain is required for subunit assembly and functional expression of this and probably other Shaker-like K+ channel proteins.