Biosynthesis of electroplax sodium channels in Electrophorus electrocytes and Xenopus oocytes.

We have synthesized the eel electroplax sodium channel core polypeptide in both a cell-free and a frog oocyte system and report it does not possess the unusual electrophoretic properties of the mature, native sodium channel polypeptide isolated from electroplax membranes. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the mature channel polypeptide exhibits both a diffuse banding pattern (microheterogeneity) and an extremely high electrophoretic free mobility. In contrast, the core polypeptide synthesized in vitro or in vivo migrates as a sharp band with a near-normal electrophoretic free mobility (Mr 230,000). The microheterogeneity of the mature peptide has been inferred to result from varying degrees of glycosylation of the channel polypeptide [Miller, J.A., Agnew, W.S., & Levinson, S.R. (1983) Biochemistry 22, 462-470]. We present evidence here that the anomalously high electrophoretic free mobility is due to the binding of large amounts of sodium dodecyl sulfate to posttranslationally modified domains on the protein. In addition, we have followed the posttranslational processing of eel sodium channels in both the eel electrocyte and the frog oocyte. Using lectin binding and Ferguson analysis, we found that the channel was processed relatively rapidly to an intermediate form in the Golgi apparatus that apparently contained fewer carbohydrate and hydrophobic domains than the mature channel. The further addition of carbohydrate and hydrophobic domains, which are required before the channel acquires its characteristic physicochemical properties, proceeded relatively slowly in the electrocyte and appeared not to have occurred to the majority of intermediately processed channels in the frog oocyte.