Sodium channel protein

F. Matsumura, sponsored by the National Institute of Environmental Health Sciences (NIEHS), plans to study the toxic effects of chlorinated and pyrethroid pesticides primarily on calcium and sodium regulating processes in the nervous system. To examine the interactions of the pesticides with calcium regulating processes, researchers will use synaptosomal preparations from the brains of rats and the central nervous systems of squid. To examine the interactions of the pesticides with sodium regulating processes, they will collect antibodies directed against sodium channel proteins. 

Figure 2.2 Diagram of a voltage-activated sodium channel protein. The channel is composed of a long chain of amino acids intercormected by peptide bonds. The amino acids perform specific functions within the ion channel. The cylinders represent amino acid assemblies located within the membrane of the nerve cell and responsible for the foundation of the ion pore.

Targeting the Sodium Channel Protein Local Anesthetics... 

Making use of the binding of radioactively labeled specific toxins to identify diem, the subunits of the sodium channel proteins were purified from several sources including die electrical tissue of the electric eel Electrophorus electricus,i37 i39 heart and skeletal muscle, and brain.440-44113 In all cases a large 260-kDa glycoprotein, which may be 30% carbohydrate, is present. The saxitoxin-binding protein from rat brain has two additional 33-36 kDa subunits witii a stoichiometry of a(31P2- The Electrophorus a subunit consists of 1820 residues,437 while rat brain contains a proteins of 2009... 

Figure 30-17 (A) Two-dimensional map of the 260-kDa a subunit of the voltage-gated Na+ channel from the electric eel Electrophorus e/ecfns.438 441 (B) Image of the sodium channel protein obtained by cryo-electron microscopy and image analysis at 1.9 nm resolution. In this side view the protein appears to be bell-shaped with a height of 13.5 nm, a square bottom (cytoplasmic surface) 10 nm on a side, and a hemispherical top with a diameter of 6.5 nm. (C) Bottom view of the protein. (D) Axial section which cuts the bottom, as viewed in (C), approximately along a diagonal. From Sato et al.438 Notice the cavities (dark) and domain structures (light). The black arrow marks a constriction between upper (extracelllar) and lower (cytoplasmic) cavities. White lines indicate approximate position of the lipid bilayer. From Sato et al.i38 Courtesy of Chikara Sato.

Figure 2. Sodium channel protein topology indicating the naturally occurring mutations that are confirmed to reduce insect sodium channel sensitivity to pyrethroids. The sodium channel contains four repeated homologous domains (I-IV), each having six membrane-spanning segments (SI-6). See the text for the details.

SCN5A Sodium channel protein type 5 subunit alpha Neuroactive drugs blocking voltage-gated sodium channels 94 76 56 16 61 62 41 40 19 14 - -... 

Comparative structural analysis of sodium channel genes has permitted the development of testable hypotheses concerning the neurotoxin recognition properties of the sodium channel protein. However, specific elements of the deduced structure have not yet been definitively correlated with the molecular recognition of sodium channel-directed neurotoxins by discrete binding domains. It is, thus, not possible at the present time to know which pharmacological properties are determined by the sodium channel protein per se and which are determined by interactions between it and crucial features of its membrane environment. Clearly, it will be necessary to analyze the effects of specific modifications of sodium channel structure in a defined membrane environment in order to address these and other questions relating to sodium channel function. 

Warashina A (1985) Frequency-dependent effects of aconitine and veratridine on membrane currents in the crayfish giant axon. Jap J Physiol 35 463-482 Weigele JB, Barchi RL (1982) Functional reconstitution of purified sodium channel protein from rat sarcolemma. Proc Natl Acad Sci USA 79 3651-3655 Yoshii M, Narahashi T (1984) Patch clamp analysis of veratridine-induced sodium channels (abstract). Biophys J 45 184a... 

The pyrethrins and pyrethrolds (permethrin) are nerve membrane sodium channel toxins that do not affect potassium channels. The compounds bind to specific sodium channel proteins and slow the rate of Inactivation of the sodium current elicited by membrane depolarization and, as a result, prolong the open time of the sodium channel. At low ooncentratlons, the pyrethrolds produce repetitive action potentials and neuron firing at high concentrations, the nerve membrane is depolarized completely and excitation blocked. 

The first report of a2,8-linked polySia on the a subunit of sodium channel proteins was by James and Agnew (1987), who identified such chains on the voltage-sensitive channel from the electric organ of the eel, Electrophorus elec-tricus. The sodium channel protein appears to be the only protein in electroplax membranes that is polysialylated, suggesting the presence of a specific a2,8-polysialyltransferase for the polysialylation of this protein (James and Agnew, 1989). 

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