BOTH N-TERMINAL AND C-TERMINAL REGIONS CONTRIBUTE TO THE ASSEMBLY ANDFUNCTIONAL EXPRESSION OF HOMOMULTIMERIC 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 mol ecularly diverse K+ channel subunits. To determine possible amino acid sequence domains required for subunit assembly and expression, we hav e constructed 15 N- and C-terminal interstitial or truncation deletion mutations in mKv1.1 (MBK1), a mouse Shaker-like K+ channel. We inject ed Xenopus oocytes with cRNA encoding each of these mutants and coinje cted each mutant cRNA with cRNA for wild-type mKv1.3, another mouse Sh aker-like K+ channel that can form heteromultinaers with mKv1.1. We fo und that the last five amino acids of the C-terminus of mKv1.1 contrib ute to functional expression by (1) rescuing the function of mutants w ith a large truncation of the C-terminus (Delta 424-495), and (2) cont ributing to the slow inactivation kinetics (time constant of 2-3 sec) of wild-type mKv1.1 whole-cell K+ currents. All C-terminal deletion mu tants were able to express at least as heteromultimers with mKv1.3, su ggesting that the C-terminus is not required for channel assembly. In contrast, nine different interstitial or truncation mutants in which p art of a highly conserved, large (80-99 amino acid residues) domain wi thin the N-terminus had been deleted were unable to express either hom omultimers or heteromultimers. The relatively small sizes and nonoverl apping distributions of the interstitial deletions enable us to sugges t that the structural integrity of this entire N-terminal domain is re quired for subunit assembly and functional expression of this and prob ably other Shaker-like K+ channel proteins.