Mj. Matunis et al., SUMO-1 MODIFICATION AND ITS ROLE IN TARGETING THE RAN GTPASE-ACTIVATING PROTEIN, RANGAP1, TO THE NUCLEAR-PORE COMPLEX, The Journal of cell biology, 140(3), 1998, pp. 499-509
RanGAP1 is the GTPase-activating protein for Ran, a small ras-like GTP
ase involved in regulating nucleocytoplasmic transport. In vertebrates
, RanGAP1 is present in two forms: one that is cytoplasmic, and anothe
r that is concentrated at the cytoplasmic fibers of nuclear pore compl
exes (NPCs). The NPC-associated form of RanGAP1 is covalently modified
by the small ubiquitin-like protein, SUMO-1, and we have recently pro
posed that SUMO-1 modification functions to target RanGAP1 to the NPC.
Here, we identify the domain of RanGAP1 that specifies SUMO-1 modific
ation and demonstrate that mutations in this domain that inhibit modif
ication also inhibit targeting to the NPC, Targeting of a heterologous
protein to the NPC depended on determinants specifying SUMO-1 modific
ation and also on additional determinants in the COOH-terminal domain
of RanGAP1. SUMO-1 modification and these additional determinants were
found to specify interaction between the COOH-terminal domain of RanG
AP1 and a region of the nucleoporin, Nup358, between Ran-binding domai
ns three and four, Together, these findings indicate that SUMO-1 modif
ication targets RanGAP1 to the NPC by exposing, or creating, a Nup358
binding site in the COOH-terminal domain of RanGAP1. Surprisingly, the
COOH-terminal domain of RanGAP1 was also found to harbor a nuclear lo
calization signal. This nuclear localization signal, and the presence
of nine leucine-rich nuclear export signal motifs, suggests that RanGA
P1 may shuttle between the nucleus and the cytoplasm.