The intracellular segment of the sodium channel b1 subunit is required for its ef®cient association with the channel a subunit

Sodium channels consist of a pore-forming a subunit and auxiliary b1 and b2 subunits. The subunit b1 alters the kinetics and voltage-dependence of sodium channels expressed in Xenopus oocytes or mammalian cells. Functional modulation in oocytes depends on speci®c regions in the N-terminal extracellular domain of b1, but does not require the intracellular C-terminal domain. Functional modulation is qualitatively different in mammalian cells, and thus could involve different molecular mechanisms. As a ®rst step toward testing this hypothesis, we examined modulation of brain NaV1.2a sodium channel a subunits expressed in Chinese hamster lung cells by a mutant b1 construct with 34 amino acids deleted from the C-terminus. This deletion mutation did not modulate sodium channel function in this cell system. Co-immunoprecipitation data suggest that this loss of functional modulation was caused by inef®cient association of the mutant b1 with a, despite high levels of expression of the mutant protein. In Xenopus oocytes, injection of approximately 10 000 times more mutant b1 RNA was required to achieve the level of functional modulation observed with injection of full-length b1. Together, these ®ndings suggest that the C-terminal cytoplasmic domain of b1 is an important determinant of b1 binding to the sodium channel a subunit in both mammalian cells and Xenopus oocytes.