Characterization of variant neuroblastoma clones with missing or altered sodium channels

Variant neuroblastoma cell clones were selected for resistance to the cytotoxic effects of neurotoxins that cause persistent activation of sodium channels. Three classes of variant clones were obtained: sodium channel-deficient clones, which have markedly reduced numbers of functional sodium channels; scorpion toxin-resistant clones, which have sodium channels with an altered interaction with scorpion toxin; and parental-type clones, which have functional sodium channels similar to the ones from N18 cells but have other heritable alterations that confer toxin resistance. The frequency of conversion to all three variant phenotypes was enhanced by treatment with the missense mutagen N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), suggesting that all three variant phenotypes are the result of mutational events. Incorporation of [35S]methionine into the alpha-subunit of the sodium channel (Mr = 270,000; pI = 5.8 +/- 0.2) was studied in normal and variant clones by affinity chromatography on wheat germ agglutinin/Sepharose followed by analysis of labeled polypeptides by 1- and 2-dimensional gel electrophoresis. Sodium channel-deficient clones do not synthesize the alpha-subunit of the sodium channel, suggesting that mutations in these clones block expression of the gene for this protein subunit. The scorpion toxin-resistant clone LV10 synthesizes an alpha-subunit which has a molecular weight and pI similar to those of the parental clones within the resolution of the methods used.