Abstract
Voltage-gated sodium channels are responsible for the upstroke of the action potential in most excitable cells, and their fast inactivation is essential for controlling electrical signaling. In addition, a noninactivating, persistent component of sodium current, I(NaP), has been implicated in integrative functions of neurons including threshold for firing, neuronal bursting, and signal integration. G-protein betagamma subunits increase I(NaP), but the sodium channel subtypes that conduct I(NaP) and the target site(s) on the sodium channel molecule required for modulation by Gbetagamma are poorly defined. Here, we show that I(NaP) conducted by Na(v)1.1 and Na(v)1.2 channels (Na(v)1.1 > Na(v)1.2) is modulated by Gbetagamma; Na(v)1.4 and Na(v)1.5 channels produce smaller I(NaP) that is not regulated by Gbetagamma. These qualitative differences in modulation by Gbetagamma are determined by the transmembrane body of the sodium channels rather than their cytoplasmic C-terminal domains, which have been implicated previously in modulation by Gbetagamma. However, the C-terminal domains determine the quantitative extent of modulation of Na(v)1.2 channels by Gbetagamma. Studies of chimeric and truncated Na(v)1.2 channels identify molecular determinants that affect modulation of I(NaP) located between amino acid residue 1890 and the C terminus at residue 2005. The last 28 amino acid residues of the C terminus are sufficient to support modulation by Gbetagamma when attached to the proximal C-terminal domain. Our results further define the sodium channel subtypes that generate I(NaP) and identify crucial molecular determinants in the C-terminal domain required for modulation by Gbetagamma when attached to the transmembrane body of a responsive sodium channel.