Random mutagenesis and functional analysis of the Ran-binding protein, RanBP1

Ran GTPase is required for nucleocytoplasmic transport of many types of car go. Several proteins that recognize Ran in its GTP-bound state (Ran GTP) po ssess a conserved Ran-binding domain (RanBD), Ran-binding protein-1 (RanBP1 ) has a single RanBD and is required for RanGAP-mediated GTP hydrolysis and release of Ran from nuclear transport receptors (karyopherins), In budding yeast (Saccharomyces cerevisiae), RanBP1 is encoded by the essential YRB1 gene; expression of mouse RanBP1 cDNA rescues the lethality of Yrb1-deficie nt cells. We generated libraries of mouse RanBP1 mutants and examined 11 mu tants in vitro and for their ability to complement a temperature-sensitive yrb1 mutant (yrb1-51(ts)) in vivo, In 9 of the mutants, the alteration was a change in a residue (or 2 residues) that is conserved in all known RanBDs , However, 4 of these 9 mutants displayed biochemical properties indistingu ishable from that of wild-type RanBP1. These mutants bound to Ran GTP, stim ulated RanGAP, inhibited the exchange activity of RCC1, and rescued growth of the yrb1-51(ts) yeast cells. Two of the 9 mutants altered in residues th ought to be essential for interaction with Ran were unable to rescue growth of the yrb1(ts) mutant and did not bind detectably to Ran in vitro. Howeve r, one of these 2 mutants (and 2 others that were crippled in other RanBP1 functions) retained some ability to coactivate RanGAP, A truncated form of RanBP1 (lacking its nuclear export signal) was able to complement the yrb1( ts) mutation. When driven from the YRB1 promoter, 4 of the 5 mutants most i mpaired for Ran binding were unable to rescue growth of the yrb1(ts) cells; remarkably, these mutants could nevertheless form ternary complexes with i mportin-5 or importin-beta and Ran-GTP, The same mutants stimulated only in efficiently RanGAP-mediated GTP hydrolysis of the Ran GTP-importin-5 comple x. Thus, the essential biological activity of RanBP1 in budding yeast corre lates not with Ran GTP binding per se or with the ability to form ternary c omplexes with karyopherins, but with the capacity to potentiate Ran-GAP act ivity toward GTP-bound Ran in these complexes.