Identification of the Biosynthetic Leukotriene C4 Export Pump in Murine Mastocytoma Cells as a Homolog of the Multidrug‐Resistance Protein

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Abstract
A membrane glycoprotein of 190 kDa has been identified previously by photoaffinity labeling as a candidate for the ATP‐dependent export pump for leukotriene C4 in mastocytoma cells [Leier, I., Jedlitschky, G., Buchholz, U. & Keppler, D. (1994) Eur. J. Biochem. 220, 599–606]. The present study indicates that this protein represents the murine homolog of the human multidrug resistance protein (MRP). In immunoblot analyses several polyclonal anti‐MRP antibodies and one monoclonal antibody recognized the protein of 190‐kDa in plasma membranes of mastocytoma cells. Immunoprecipitation after photoaffinity labeling with [3H]leukotriene C4 precipitated the labeled 190‐kDa glycoprotein. Deglycosylation by glycopeptide N‐glycosidase F of mastocytoma membrane proteins was performed in comparison with membranes from MRP‐overexpressing cells and resulted in a reduction of the molecular mass of 190 kDa by about 20 kDa in all membrane preparations. The expression of the murine mrp gene in the mastocytoma cells was analyzed by amplification and sequencing of two mrp cDNA fragments in the first nucleotide binding domain (182 bp) and in a domain proximal to the 3′‐end (291 bp). The deduced amino acid sequences of these fragments were identical with murine Mrp and 86.7% and 89.7% identical with the corresponding sequences of human MRP. These results indicate that the ATP‐dependent release of leukotriene C4 by murine mastocytoma cells is mediated by murine Mrp.

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