Torpedo subunits

Skeletal muscle (muscle nAChR, comprised of al, 31,y or e, and 8 subunits, also present in electric tissues of Torpedo and Electrophorus)... 

Radioiodinated derivatives have been prepared to define more closely the target site of a-conotoxins on the acetylcholine receptor (R. Myers, unpublished data). In membrane preparations from Torpedo electroplax, photoactivatable azidosalicylate derivatives of a-conotoxin GIA preferentially label the p and 7 subunits of the acetylcholine receptor. However, when the photoactivatable derivative is cross-linked to detergent solubilized acetylcholine receptor (AChR), only the 7 subunit is labeled. Since snake a-neurotoxins mainly bind to the a subunits of AChR and a-conotoxins compete directly with a-bungarotoxin, the cross-linking results above are both intriguing and problematic. 

The P subunit is not as well conserved as the a subunit, with 91 % overall homology between the P subunit of sheep, pig, and human and 61% between the p subunit of human and Torpedo. Homologies between -subunit isoforms and the p subunit of H,K-ATPase are moderate, 25-30%, but some segments are well conserved. As shown in Table II, beta signatures 1 and 2 are preserved in the (6-subunit isoforms and P subunit of H,K-ATPase [70] as an indication that the positions of tryptophans and cysteines are well conserved elements. 

The acetylcholine receptor (AChR) of Torpedo electric organ is also a PCP "receptor." However, this nicotinic AChR has about one-tenth the affinity for PCP than that of the rat brain PCP receptor [K0.5 = 0.3 pM, versus = 4-6 pM for Torpedo (Heidmann et al. 1983 flaring et al. 1984)]. Moreover, the nicotinic AChR has subunits of MR<66 kD, and these are the subunits that are specifically labelled with 3H-Az-PCP in the Torpedo electroplax membranes (Heidmann et al. 1983 Haring and Kloog 1984 Haring et al. 1984). These data indicate that the nicotinic AChR-PCP receptor differs from the rat brain PCP receptor. Furthermore, our findings are... 

Haring, R. Kloog, Y. and Sokolvsky, M. Localization of phencyclidine binding sites on a and B subunits of the nicotinic acetylcholine receptor from Torpedo ocellata electric organ using azido phencyclidine. J. Neurosci 34-1047-1055, 1984. 

The ability of a receptor channel to conduct ions, which is measured as the conductance (the reciprocal of resistance) of the channel, depends on TM2. The experiments that showed this were based on the observation that receptors made of Torpedo a, p, v, and 8 subunits had a different conductance from receptors made of Torpedo a, p, y, and calf 6 subunits. When chimeric 8 subunits in which parts of the Torpedo sequences were replaced by the corresponding calf sequence, were... 

Xu et al. [5] described the effect of (z>)-penicillamine on the binding of several antiacetylcholine receptor monoclonal antibodies to the Torpedo acetylcholine receptor. Penicillamine is covalently incorporated into the acetylcholine receptor through SS exchange at the cysteine residues of the a-subunit, altering the antigenic structure of the receptor. This effect on the structure of the native receptor at the neuromuscular junction may be responsible for the establishment of the autoimmune response to the acetylcholine receptor in (i))-penicillamine-induced myasthenia gravis. Cysteine and penicillamine interact to form penicillamine-cysteine mixed disulfide complexes [6] ... 

Chiara DC, Middleton RE, Cohen JB. 1998. Identification of tryptophan 55 as the primary site of [ H] nicotine photoincorporation in the y-subunit of the Torpedo nicotinic acetylcholine receptor. FEBS Lett 423 223-226. 

M. Noda, H. Takahashi, T. Tanabe, M. Toyosato, Y. Enrutani, T. Hirose, M. Asak, S. Inayama, T. Miyata, S. Numa (1982). Primary structure of a-subunit precursor of Torpedo califomica acetylchohne receptor from cDNA sequence. Nature 299 793-802. 

Pentameric structure in Torpedo electric organ and fetal mammalian muscle has two G.1 subunits and one each of Bl, 5, and 7 subunits. The stoichiometry is indicated by subscripts, eg, [(ct-l)2 Bl 5 7]. In adult muscle, the 7 subunit is replaced by an ... 

Noda, M. et al. (1982). primary structure of the a subunit precursor of Torpedo acetylcholine receptor deduced from cDNA sequence. Nature (London) 299, 793-797. 

Noda M, Takahashi H, Tanabe T, Toyosato M, Eurutani Y, Hirose T, Asai M, Inayama S, Miyata T, Numa S. Primary structure of alpha-subunit precursor of Torpedo califomica acetylchoUne receptor deduced from cDNA sequence. Nature 1982 299 793-797. 

Figure 5. Model of the proposed acetylcholine recognition site on the alpha subunit of the Torpedo ACh receptor in the region between Cys 128 and Cys 142. An ACh molecule is shown in relation to the four residues postulated to interact in its binding. Reprinted with permission from Ref. 9. Copyright 1985 Elsevier Science Publishers.

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