Expression of functional sodium channels from cloned cDNA

Abstract

The voltage-gated sodium channel is a transmembrane protein essential for the generation of action potentials in excitable cells¹. It has been reported that sodium channels purified from the electric organ of the electric eel, Electrophorus electricus^2,3, and from chick cardiac muscle⁴ consist of a single polypeptide of relative molecular mass (M_r)∼ 260,000, whereas those purified from rat brain⁵ and from rat^6,7 and rabbit skeletal muscle⁷ contain, in addition to the large polypeptide, one or two smaller polypeptides of M_r33,000–43,000. The primary structures of the Electrophorus sodium channel⁸ and two distinct sodium channel large polypeptides⁹ (designated as sodium channels I and II) from rat brain have been elucidated by cloning and sequencing the complementary DNAs. The purified sodium channel preparations from Electrophorus electroplax¹⁰ and from mammalian muscle^11,12 and brain^13–15, when reconstituted into lipid vesicles or planar lipid bilayers, exhibit some functional activities. The successful reconstitution with the Electrophoruspreparation would imply that the large polypeptide alone is sufficient to form functional sodium channels. However, studies with the rat brain preparation suggest that the smaller polypeptide of M_r36,000 is also required for the integrity of the saxitoxin (STX) or tetrodotoxin (TTX) binding site of the sodium channel¹⁶. Here we report that the messenger RNAs generated by transcription of the cloned cDNAs encoding the rat brain sodium channel large polypeptides, when injected into Xenopus oocytes, can direct the formation of functional sodium channels.