Ran is a nuclear Ras-like GTPase that is required for the bidirectional transport of proteins and ribnucleoproteins across the nuclear pore complex (NPC). A key regulator of the Ran GTP/GDP cycle is the 70-kD Ran-GTPase-activating protein RanGAP1. Here, we report the identification and localization of a novel form of RanGAP1. Using peptide sequence analysis and specific mAbs, RanGAP1 was found to be modified by conjugation to a ubiquitin-like protein. Immunoblot analysis and immunolocalization by light and EM demonstrated that the 70-kD unmodified from of RanGAP1 is exclusively cytoplasmic, whereas the 90-kD modified form of RanGAP1 is associated with the cytoplasmic fibers of the NPC. The modified form of RanGAP1 also appeared to associated with the mitotic spindle apparatus during mitosis. These findings have specific implications for Ran function and broad implications for protein regulation by ubiquitin-like modifications. Moreover, the variety and function of ubiquitin-like protein modifications in the cell may be more diverse than previously realized.

As the sole site of nucleocytoplasmic transport, the nuclear pore complex (NPC) has a vital cellular role. Nonetheless, much remains to be learned about many fundamental aspects of NPC function. To further understand the structure and function of the mammalian NPC, we have completed a proteomic analysis to identify and classify all of its protein components. We used mass spectrometry to identify all proteins present in a biochemically purified NPC fraction. Based on previous characterization, sequence homology, and subcellular localization, 29 of these proteins were classified as nucleoporins, and a further 18 were classified as NPC-associated proteins. Among the 29 nucleoporins were six previously undiscovered nucleoporins and a novel family of WD repeat nucleoporins. One of these WD repeat nucleoporins is ALADIN, the gene mutated in triple-A (or Allgrove) syndrome. Our analysis defines the proteome of the mammalian NPC for the first time and paves the way for a more detailed characterization of NPC structure and function.

The PML protein, identified first as part of the oncogenic PML–RARα chimera in acute promyelocytic leukemia (APL), concentrates within discrete subnuclear structures, corresponding to some types of nuclear bodies. These structures are disrupted in APL cells, and retinoic acid (RA) can trigger their reorganization, correlating with its therapeutic effect in this type of leukemia. Recently, arsenic trioxide (As2O3) was identified as a potent antileukemic agent which, similarly to RA, induces complete remissions in APL patients. Here we show that, in APL cells, As2O3 triggers rapid degradation of PML–RARα and provokes the restoration of intact nuclear bodies. In non‐APL cells, the ubiquitin‐like protein SUMO‐1 is covalently attached to a subset of wild‐type PML in a reversible and phosphorylation‐dependent manner. The unmodified form of PML is found in the soluble nucleoplasmic fraction, whereas the SUMO‐1‐polymodified forms of PML are compartmentalized exclusively in the PML nuclear bodies. As2O3 administration strikingly increases the pool of SUMO‐1–PML conjugates that, subsequently, accumulate in enlarged nuclear bodies. In contrast to PML–RARα, the overall amount of PML seems to remain unaltered up to 36 h following As2O3 treatment. These findings indicate that the conjugation of PML with SUMO‐1 modulates its intracellular localization and suggest that post‐translational modification by SUMO‐1 may be more generally involved than previously suspected in the targeting of proteins to distinct subcellular structures. They provide additional evidence that the role of ‘ubiquitin‐like’ post‐translational modification is not limited to a degradation signal.

The Ras-related nuclear protein, Ran, has been implicated in nuclear transport. By screening a HeLa cell lambda expression library with Ran-GTP and sequencing overlapping cDNA clones, we have obtained the derived primary structure of a protein with a calculated molecular mass of 358 kDa. Using antibodies raised against an expressed segment of this protein, we obtained punctate nuclear surface staining by immunofluorescence microscopy that is characteristic for nucleoporins. Electron microscopy of immunogold-decorated rat liver nuclear envelopes sublocalized the 358-kDa protein at (or near) the tip of the cytoplasmic fibers of the nuclear pore complex (NPC). In agreement with current convention, this protein was therefore termed Nup358 (for nucleoporin of 358 kDa). Nup358 contains a leucine-rich region, four potential Ran binding sites (i.e. Ran binding protein 1 homologous domains) flanked by nucleoporin-characteristic FXFG or FG repeats, eight zinc finger motifs, and a C-terminal cyclophilin A homologous domain. Consistent with the location of Nup358 at the cytoplasmic fibers of the NPC, we found decoration with Ran-gold at only the cytoplasmic side of the NPC. Thus, Nup358 is the first nucleoporin shown to contain binding sites for two of three soluble nuclear transport factors so far isolated, namely karyopherin and Ran-GTP.