TRUNCATED ERYTHROPOIETIN RECEPTOR IN A MURINE ERYTHROLEUKEMIA CELL-LINE

The Friend spleen focus forming virus produces a 55 kDa envelope glyco protein which associates with the erythropoietin receptor. We compared the erythropoietin receptor in Friend virus transformed murine erythr oleukemic F4N and 707 cell lines with the J2E erythroid line generated by the J2 retrovirus. Reverse transcriptase PCR was used to determine transcript size. Erythropoietin receptor cDNAs were then sequenced an d protein products analysed by Western blotting and immunoprecipitatio n. We show here that the F4N marine erythroleukemic cell line had an e nlarged erythropoietin receptor mRNA. In contrast, the 707 and J2E cel l lines had normal sized transcripts for the receptor. Sequence analys is of the receptor in F4N cells revealed that introns which separate t he exons coding for the cytoplasmic domain of the receptor were retain ed in these transcripts. As a consequence, a premature stop codon had been introduced, leaving only four amino acids in the intracellular po rtion of the receptor molecule. The normal erythropoietin receptor is approx. 66-70kDa, bat immunoprecipitation of [S-35]methionine/cysteine labelled cell lysates with an antibody to the amino-terminus of the e rythropoietin receptor identified a truncated 37 kDa protein in F4N ce lls. Despite the severe carboxy-terminal truncation of the erythropoie tin receptor, F4N cells continued to proliferate like the other murine erythroleukemia cell lines. This study shows that failure to remove i ntrons from the erythropoietin receptor mRNA in F4N cells has resulted in the production of a smaller protein with virtually no cytoplasmic domain.