A family ofRanbinding proteins thatincludes nucleoporins

Ran,asmall nuclear GTPbinding protein, is essential forthetranslocation ofnuclear proteins through the nuclear porecomplex. Weshowthat several proteins, includ- ingtheSaccharomyces cerevisiae Nup2pandCaenorhabditis elegans F59A2.1 nucleoporins, contain domains similar tothe previously characterized murine Ranbinding protein (RBP, termed RBP1). Totest thesignificance ofthis similarity, we haveusedthecorresponding domains ofNup2pandaputative S.cerevisiae RBPinRanbinding assays andtheyeast two- hybrid system. Bothproteins bindS.cerevisiae Ran,butonly theputative S.cerevisiae RBPbinds humanRan.Two-hybrid analysis revealed Ran-Raninteractions andthatyeast and humanRanscaninteract. Thesedataidentify Nup2pasa target forRaninthenuclear porecomplex, suggesting adirect roleforitinnuclear-cytoplasmic transport. Wediscuss the possibility thatproteins harboring Ranbinding domains link theRanGTPasecycle tospecific functions inthenucleus. Nuclear protein import proceeds intwomajorsteps, an energy-independent