Shaker-related voltage-gated K+ (K-v) channels(1,2) are assembled from ion-conducting K-v alpha subunits, which are integral membrane proteins, and auxiliary K-v beta subunits. This leads to the formation of highly diverse heteromultimeric K-v channels that mediate outward currents with a wide range of time courses for inactivation, Two principal inactivation mechanisms have been recognized(1) : C-type inactivation correlated with carboxy-terminal K-v alpha-subunit structures(3), and N-type inactivation conferred by ＇ball＇ domains in the amino termini of certain K-v alpha(4,5) and K-v beta(6) subunits, Assembly of heteromultimers with one or more K-v alpha(4,7)- and/or K-v beta(6) ball domains appears to be an essential principle of the generation of A-type K-v channel diversity, Here we show that, unexpectedly, the presence of K-v alpha- or K-v beta-ball domains does not dominate the gating phenotype in heteromultimers containing K(v)1.6 alpha subunits. These heteromultimers mediate non-inactivating currents because of the dominant-negative activity of a new type of N-type inactivation-prevention (NIP) domain present in the K(v)1.6 amino terminus. Mutations in the NIP domain lead to loss of function, and its transfer to another K-v alpha subunit leads to gain of function. Our discovery of the NIP domain, which neutralizes the activity of K-v alpha- and K-v beta-inactivation gates, establishes a new determinant for the gating behaviour of heteromultimeric K-v channels.