TISSUE-SPECIFIC EXPRESSION OF RAN ISOFORMS IN THE MOUSE

Ran genes encode a family of well-conserved small nuclear GTPases (Ras -related nuclear proteins), whose function is implicated in both norma l cell cycle progression and the transport of RNA and proteins between the nucleus and the cytoplasm. Previous studies of Ran proteins have utilized cell-free systems, yeasts, and cultured mammalian cells. We h ave now characterized patterns of Ran gene expression in the mouse. Se rum starvation suppressed Ran gene transcription in mouse 3T3 cells. R an mRNA reappeared in cells within 3 h after refeeding. A single Ran m RNA species was detected at low levels in most somatic tissues of the adult mouse. In testis, this Ran mRNA was abundant, as were other larg er transcripts. Analysis of testis-derived Ran cDNA clones revealed th e presence of two transcripts, one specifying an amino acid sequence i dentical to that of human Ran/TC4 and one specifying an amino acid seq uence 94% identical. Northern blotting and reverse transcriptase-PCR a ssays with oligonucleotide probes and primers specific for each transc ript demonstrated that the isoform identical to Ran/TC4 was expressed in both somatic tissues and testis, while the variant form was transcr ibed only in testis. The existence of tissue-specific Ran isoforms may help to rationalize the diverse roles suggested for Ran by previous b iochemical studies.