<Review>Biochemical Study on Nicotinic Acetylcholine Receptor

Nicotinic acetylcholine receptor (AChR) is a multisubunit membrane glycoprotein which functions as a ligand-triggered cation channel. Receptors from electric tissues and skeletal muscle have a molecular weight of approximately 290,000 as a monomer and are composed of four types of polypeptide chains which assemble into a heterologous α_2βγδ pentamer. We determined the carbohydrate structure of Torpedo AChR, which was the first determination for neurotransmitter receptors. About 70 % of the oligosaccharides of the AChR were of the high mannose-type, Man_9GlcNAc_2 and Man_8GlcNAc_2. These two types of oligosaccharides were distributed in all the subunits. The remaining were various kinds of complex-type oligosaccharides, existing mainly in the γ and δ subunits. The α and β subunits had only one carbohydrate chain each, while the γ and δ subunits had two and three carbohydrate chains, respectively. These glycosylation sites were identified by sequencing glycopeptides obtained by lectin-affinity chromatography. The participation of oligosaccharides in ligand-binding of AChR was examined using a newly developed binding assay. The sialic acids and high mannose-type oligosaccharides on AChR were found to be unnecessary for its ligand binding. Next we found that the β and δ subunits of Torpedo AChR were phosphorylated on their tyrosine residues. The level of the phosphorylation was enhanced by incubating the AChR-rich membrane fraction with cholinergic ligands. This suggests that cholinergic agonists physiologically regulate phosphorylation on tyrosine in vivo, which might be included in the desensitization mechanism of the receptor. We also examined the spatial relation of proteins surrounding AChR using Torpedo AChR-rich membrane fraction. Bifunctional crosslinkers revealed an intimate relation among the AChR γ subunit, 43-kD protein, and dystrophin. Finally, we found neurotoxin-binding activities in the su-pernatant fraction obtained by ultracentrifugation of a homogenate of the electric organ, which usually does not contain AChR. This new activity was different in nature from AChR, and could function as a regulator or a modulator for AChR function

Advanced burning stages and fate of 8-10 Mo stars

The stellar mass range 8<M/Mo<12 corresponds to the most massive AGB stars and the most numerous massive stars. It is host to a variety of supernova progenitors and is therefore very important for galactic chemical evolution and stellar population studies. In this paper, we study the transition from super-AGB star to massive star and find that a propagating neon-oxygen burning shell is common to both the most massive electron capture supernova (EC-SN) progenitors and the lowest mass iron-core collapse supernova (FeCCSN) progenitors. Of the models that ignite neon burning off-center, the 9.5Mo model would evolve to an FeCCSN after the neon-burning shell propagates to the center, as in previous studies. The neon-burning shell in the 8.8Mo model, however, fails to reach the center as the URCA process and an extended (0.6 Mo) region of low Ye (0.48) in the outer part of the core begin to dominate the late evolution; the model evolves to an EC-SN. This is the first study to follow the most massive EC-SN progenitors to collapse, representing an evolutionary path to EC-SN in addition to that from SAGB stars undergoing thermal pulses. We also present models of an 8.75Mo super-AGB star through its entire thermal pulse phase until electron captures on 20Ne begin at its center and of a 12Mo star up to the iron core collapse. We discuss key uncertainties and how the different pathways to collapse affect the pre-supernova structure. Finally, we compare our results to the observed neutron star mass distribution.Comment: 20 pages, 14 figures, 1 table. Submitted to ApJ 2013 February 19; accepted 2013 June