Biomedical subjects
Publications and source records attributed to S Numa.
The Torpedo and calf acetylcholine receptors and hybrids composed of subunits from the two species have been produced in Xenopus oocytes by the use of the cloned complementary DNAs. Single-channel current measurements indicate that these receptors form channels of similar conductance but with different gating behaviour.
The sodium channel is a voltage-gated ionic channel essential for the generation of action potentials. It has been reported that the sodium channels purified from the electric organ of Electrophorus electricus (electric eel) and from chick cardiac muscle consist of a single polypeptide of relative molecular mass (Mr) approximately 260,000 (260K), whereas those purified from rat brain and skeletal muscle contain, in addition to the large polypeptide, two or three smaller polypeptides of Mr 37-45K. Recently, we have elucidated the primary structure of the Electrophorus sodium channel by cloning and sequencing the DNA complementary to its messenger RNA. Despite the apparent homogeneity of the purified sodium channel preparations, several types of tetrodotoxin (or saxitoxin) binding sites or sodium currents have been observed in many excitable membranes. The occurrence of distinguishable populations of sodium channels may be attributable to different states of the same channel protein or to distinct channel proteins. We have now isolated complementary DNA clones derived from two distinct rat brain mRNAs encoding sodium channel large polypeptides and present here the complete amino-acid sequences of the two polypeptides (designated sodium channels I and II), as deduced from the cDNA sequences. A partial DNA sequence complementary to a third homologous mRNA from rat brain has also been cloned.
Distinct classes of acetylcholine receptor channels are formed when Xenopus oocytes are injected with combinations of the bovine alpha-, beta-, gamma- and delta- or the alpha-, beta-, gamma- and epsilon-subunit-specific messenger RNAs. The conductance and gating properties of the two classes of channels, in conjunction with the developmental changes in the muscular contents of the mRNAs, suggest that replacement of the gamma-subunit by the epsilon-subunit is responsible for the functional alteration of the receptor during muscle development.
The voltage-gated sodium channel is a transmembrane protein essential for the generation of action potentials in excitable cells. It has been reported that sodium channels purified from the electric organ of the electric eel, Electrophorus electricus, and from chick cardiac muscle consist of a single polypeptide of relative molecular mass (Mr) approximately 260,000, whereas those purified from rat brain and from rat and rabbit skeletal muscle contain, in addition to the large polypeptide, one or two smaller polypeptides of Mr 33,000-43,000. The primary structures of the Electrophorus sodium channel and two distinct sodium channel large polypeptides (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 electroplax and from mammalian muscle and brain, when reconstituted into lipid vesicles or planar lipid bilayers, exhibit some functional activities. The successful reconstitution with the Electrophorus preparation 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 Mr 36,000 is also required for the integrity of the saxitoxin (STX) or tetrodotoxin (TTX) binding site of the sodium channel. 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.
The muscarinic acetylcholine receptor (mAChR) mediates various cellular responses, including inhibition of adenylate cyclase, breakdown of phosphoinositides and modulation of potassium channels, through the action of guanine nucleotide-binding regulatory proteins. Pharmacologically distinguishable forms of the mAChR occur in different tissues and have provisionally been classified into M1 (I), M2 cardiac (II) and M2 glandular (III) subtypes on the basis of their difference in apparent affinity for antagonists. In an attempt to elucidate the molecular basis of the functional heterogeneity of the mAChR, we have cloned and sequenced DNAs complementary to porcine cerebral and cardiac messenger RNAs encoding mAChRs and have thereby deduced the primary structures of the receptor proteins. We report here that the messenger RNA generated by transcription of the cardiac complementary DNA directs the formation of a functional mAChR in Xenopus oocytes and that this mAChR differs from the mAChR formed by expression of the cerebral cDNA both in acetylcholine (ACh)-induced response and in antagonist binding properties. Our results provide evidence indicating that the mAChR encoded by the cerebral cDNA (designated as mAChR I) and the mAChR encoded by the cardiac cDNA (mAChR II) are of the M1 (I) and the M2 cardiac (II) subtype, respectively.
The complete amino-acid sequence of the receptor for dihydropyridine calcium channel blockers from rabbit skeletal muscle is predicted by cloning and sequence analysis of DNA complementary to its messenger RNA. Structural and sequence similarities to the voltage-dependent sodium channel suggest that in the transverse tubule membrane of skeletal muscle the dihydropyridine receptor may act both as voltage sensor in excitation-contraction coupling and as a calcium channel.
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