Voltage-sensitive sodium channels are responsible for the initiation and propagation of the action potential and therefore are important for neuronal excitability. Complementary DNA clones encoding the beta 1 subunit of the rat brain sodium channel were isolated by a combination of polymerase chain reaction and library screening techniques. The deduced primary structure indicates that the beta 1 subunit is a 22,851-dalton protein that contains a single putative transmembrane domain and four potential extracellular N-linked glycosylation sites, consistent with biochemical data. Northern blot analysis reveals a 1,400-nucleotide messenger RNA in rat brain, heart, skeletal muscle, and spinal cord. Coexpression of beta 1 subunits with alpha subunits increases the size of the peak sodium current, accelerates its inactivation, and shifts the voltage dependence of inactivation to more negative membrane potentials. These results indicate that the beta 1 subunit is crucial in the assembly, expression, and functional modulation of the heterotrimeric complex of the rat brain sodium channel.

Voltage-gated sodium channels in brain neurons are complexes of a pore-forming alpha subunit with smaller beta 1 and beta 2 subunits. cDNA cloning and sequencing showed that the beta 2 subunit is a 186 residue glycoprotein with an extracellular NH2-terminal domain containing an immunoglobulin-like fold with similarity to the neural cell adhesion molecule (CAM) contactin, a single transmembrane segment, and a small intracellular domain. Coexpression of beta 2 with alpha subunits in Xenopus oocytes increases functional expression, modulates gating, and causes up to a 4-fold increase in the capacitance of the oocyte, which results from an increase in the surface area of the plasma membrane microvilli. beta 2 subunits are unique among the auxiliary subunits of ion channels in combining channel modulation with a CAM motif and the ability to expand the cell membrane surface area. They may be important regulators of sodium channel expression and localization in neurons.

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Voltage-gated L-type Ca2+ channels mediate Ca2+ entry into cells in response to membrane depolarization. Ca2+ entry through the cardiac Ca2+ channel determines the rate and force of contraction, and modulation of Ca2+ channel activity by beta-adrenergic agents acting through adenosine 3',5'-cyclic monophosphate-(cAMP)-dependent protein phosphorylation contributes to physiological regulation of cardiac function by the sympathetic nervous system. Immunoblotting experiments using site-directed anti-peptide antibodies against different peptide segments indicate that the alpha 1 subunit of the cardiac L-type Ca2+ channel exists in two size forms with apparent molecular masses of 240 and 210 kDa, which we call alpha 1(242) and alpha 1(210), Alpha 1(242) corresponds to the full-length cardiac alpha 1 subunit predicted from its cDNA sequence, while alpha 1(210) is truncated at its COOH terminus. Only alpha 1(242) is phosphorylated in vitro by cAMP-dependent protein kinase. Protein microsequencing and peptide mapping of wild-type and mutant fusion proteins show that this phosphorylation occurs at serine 1928 near the COOH terminus. Phosphorylation of this residue can be detected by phosphospecific antibodies raised against the corresponding phosphopeptide. Experiments with these antibodies show that alpha 1(242) is phosphorylated in intact cells expressing the cardiac alpha 1 subunit in response to increased intracellular levels of cAMP. These results identify serine 1928 on the alpha 1 subunit as a possible site of regulation by cAMP-dependent phosphorylation.