Cell-specific Posttranslational Events Affect Functional Expression at the Plasma Membrane but not Tetrodotoxin Sensitivity of the Rat Brain IIA Sodium Channel a-Subunit Expressed in Mammalian Cells

The rat brain IIA Na+ channel a-subunit was expressed and studied in mammalian cells. Cells were infected with a recombinant vaccinia virus (VV) carrying the bacteriophage T7 RNA polymerase gene and were transfected with cDNA encoding the IIA Na+ channel a-subunit under control of a T7 promoter. Whole-cell patch-clamp recording showed that functional IIA channels were expressed efficiently (10 channels/pm* in -60% of cells) in Chinese hamster ovary (CHO) cells and in neonatal rat ventricular myocytes but were expressed poorly in undifferentiated BC,Hl cells and failed to express in Ltkcells. However, voltage-dependent Drosophila Shaker H4 K+ channels and Escherichia co/i &galactosidase were expressed efficiently in all four cell types with VV vectors. Because RNA synthesis probably occurs without major differences in the cytoplasm of all infected cell types under the control of the T7 promoter and T7 polymerase, we conclude that cell type-specific expression of the Na+ channel probably reflects differences at posttranslational steps. The gating properties of the IIA Na+ currents expressed in cardiac myocytes differed from those expressed in CHO cells; most noticeably, the IIA Na+ currents displayed more rapid macroscopic inactivation when expressed in cardiac myocytes. These differences also suggest cell-specific posttranslational modifications. IIA channels were blocked by -90% by 90 nh! TTX when expressed either in CHO cells or in cardiac myocytes; the latter also continued to display endogenous TTX-resistant Na+ currents. Therefore, the TTX binding site of the channel is not affected by cell-specific modifications and is encoded by the primary amino acid sequence.