A vector for the synthesis of cRNAs encoding Myc epitope-tagged proteins in Xenopus laevis oocytes

We describe a plasmid, pNKS2-myc, designed for convenient in-frame fusion of an antibody-specific epitope sequence to the N terminus of a desired cDNA and subsequent synthesis of transcripts that direct the synthesis of the tagged polypeptide in Xenopus laevis (Xl) oocytes. pNKS2-myc contains an SP6 promoter, followed by the translation initiation sequence of the Na,K-pump beta 3 subunit of Xl and the sequence encoding an epitope derived from the human c-myc proto-oncogene product. Appropriate restriction sites allow one to insert virtually any desired cDNA fragment directly behind the epitope-specific sequence and before a long poly(A) tail. After linearization with EcoRI or NotI, polyadenylated cRNA can be synthesized that is efficiently translated in Xl oocytes. The utility of pNKS2-myc is demonstrated by cloning cDNAs coding for Na,K-pump subunits into this vector and injecting the corresponding cRNAs into oocytes. The tagged mouse beta 1 and beta 2 subunit isoforms could be purified from detergent extracts of these cells by immunoprecipitation with a generally available monoclonal antibody (mAb) to the tag, 9E10, as well as with specific mAb that recognize individual beta subunit isoforms. Under native conditions, endogenous and coexpressed exogenous alpha 1 subunits (the catalytic subunit of the Na,K-pump) were co-precipitated, indicating that the N-terminal addition of the decapeptide epitope has no adverse effect on the folding of beta subunits nor on their assembly with alpha subunits. Furthermore, the Myc-specific mAb likewise precipitated a Myc-tagged Na,K-pump alpha 1 subunit together with any of the co-synthesized beta subunits.