TEA binding sites in crayfish axon membranes

Abstract

VOLTAGE-DEPENDENT ion channels are the basis of nerve impulse propagation along the axon. The mechanism of gating and ionic selectivity of these channels remains obscure as long as the membrane molecules participating in these processes are unknown. Some biochemical properties of a tetrodotoxin binding component of the sodium channel have been determined¹, but for the potassium channel not even the number of channels per unit axon surface is known. From noise measurements it has been estimated that the squid giant axon contains about 60 K⁺ channels μm⁻² (ref. 2), but this figure remains to be confirmed by other methods. Here I report experiments showing that specific and covalent labelling of tetraethylammonium (TEA) binding sites with a radioactive photoaffinity label can be achieved. 4-Azido-2-nitrobenzyltriethylammonium fluoroborate has been shown to block selectively, and after irradiation irreversibly, potassium conductance in the node of Ranvier of frog sciatic nerve³. I present here evidence that in crayfish unmyelinated nerve fibres, the label used reacts with a hydrophobic area at the mouth of the potassium channel postulated by Armstrong⁴.