Alternative splicing at the MEFV locus involved in familial Mediterranean fever regulates translocation of the marenostrin/pyrin protein to the nucleus

Mutations in MEFV, a gene encoding a protein (marenostrin/pyrin) of unknown function, are associated with familial Mediterranean fever, a genetic cond ition characterized by febrile episodes of serosal inflammation. Based on i ts primary structure, this 781 residue protein is thought to function as a nuclear effector molecule. However, recent transient expression studies ind icated a perinuclear cytoplasmic localization. Here, we describe the isolat ion and expression of a novel human MEFV isoform, MEFV-d2, generated by in- frame alternative splicing of exon 2. This transcript, expressed in leukocy tes, predicts a 570 residue protein designated marenostrin-d2. To investiga te differences in subcellular focalization between the full-length protein (marenostrin-fl) and marenostrin-d2, while providing against the overexpres sion of transiently expressed proteins, we have generated CHO cell lines st ably expressing these two isoforms fused to the green fluorescent protein. The localization pattern of marenostrin-d2 differs dramatically from that o f marenostrin-fl. Marenostrin-fl is homogeneously distributed over the enti re cytoplasm, whereas marenostrin-d2 concentrates into the nucleus. To map the critical domain(s) specifying these differences, deletion mutants have been generated. Deletion of the putative nuclear localization signals (NLS) does not alter the nuclear localization of marenostrin-d2 whereas, despite the lack of discernible NLS in the domain encoded by the exon 1-exon 3 spl ice junction, deletion of this domain indeed disrupts this localization. Th ese data, which challenge the current domain organization model of marenost rin, strongly suggest that MEFV encodes a nuclear protein and raises the po ssibility that MEFV alternative splicing may control functions of wild-type and mutant marenostrin proteins by regulating their translocation to the n ucleus.