Full SUMO System at the Nuclear Envelope

Science's STKE  15 Jan 2002:
Vol. 2002, Issue 115, pp. tw26
DOI: 10.1126/stke.2002.115.tw26

SUMO E3 ligases are showing up at the nuclear envelope now. Pichler et al. report that RanBP, a docking component of the nucleocytoplasmic transport system, fulfills the criteria for being an E3 ligase. Many SUMO targets require the presence of a nuclear localization signal. However, it has not been clear whether these targets require entry into the nucleus before SUMOylation. SUMO modification of nuclear envelope proteins was suggested by the requirement for SUMO E1 and E2 enzymes in the accumulation of fluorescently labeled SUMO at the nuclear membrane. This corroborates previous reports that RanBP2 and RanGAP, another nuclear pore complex protein, are modified by SUMO and that a SUMO E2 ligase associates with SUMOylated RanGAP in vitro and in vivo. RanBP2 also stimulated the SUMO modification of the nuclear protein sp100 in vitro, but not of all known SUMO targets, a specificity that is characteristic of E3 ligases. RanBP2 itself was modified in vitro by chains of SUMO molecules. The authors also show that RanBP2 interacts directly with the E2 ligase and promotes SUMOylation of sp100 by stimulating SUMO transfer from E2 to the substrate. RanBP2 itself did not directly interact with sp100. The authors propose that the dual role of RanBP2 in nuclear import and SUMOylation correlates well with the nuclear localization signal requirement for SUMO modification of several target proteins in vivo. It is possible that this modification system may regulate nuclear translocation of proteins or serve to switch proteins to a nuclear mode of action.

A. Pichler, A. Gast, J. S. Seeler, A. Dejean, F. Melchior, The nucleoprotein RanBP2 has SUMO1 E3 ligase activity. Cell 108, 109-120 (2002). [Online Journal]