A SINGLE AMINO-ACID DETERMINES THE SUBUNIT-SPECIFIC SPIDER TOXIN BLOCK OF PHA-AMINO-3-HYDROXY-5-METHYLISOXAZOLE-4-PROPIONATE KAINATE RECEPTOR CHANNELS

Joro spider toxin (JSTX) is one of the most potent antagonists of glut amatergic AMPA/KA -3-hydroxy-5-methylisoxazole-4-propionate/kainate) r eceptor channels in invertebrates and vertebrates. A differential bloc king effect on certain types of glutamatergic synapses-e.g., parallel and climbing fiber synaptic inputs to rat cerebellar Purkinje neurons- has been shown by using a synthetic analog of the spider toxin. By inv estigating the molecular basis of the JSTX action on the recombinant A MPA/KA receptors GluR1-GluR4 and GluR6 expressed in Xenopus oocytes, w e found that submicromolar concentrations of JSTX exert a subunit-spec ific block. Thus, receptor subunits forming a receptor channel with a linear current-voltage (I-V) relationship (GluR1/2, GluR2/3, and GluR6 ) were not affected, while receptor subunits with rectifying I-V relat ionships (GluR1, GluR3, GluR4, and GluR1/3) were reversibly blocked by JSTX. By using receptor-subunit mutants obtained by site-directed mut agenesis, we have identified a single amino acid position (glutamine i n the proposed second transmembrane domain) that is critical for the J STX block. Since this site has previously been shown to control the I- V relationship of the AMPA/KA receptor channel and to participate in t he regulation of the channel's permeability for calcium ions, our find ings suggest that JSTX binds dose to the central pore region of the ch annel.