SUMO-1 - UBIQUITIN GAINS WEIGHT

The highly conserved ubiquitin polypeptide functions by covalently mod ifying other proteins. This modification has a well-established role i n facilitating substrate degradation by the proteasome and can regulat e some proteins by ways other than targeting them to the proteasome. I t has now emerged that proteins bearing only distant similarity to ubi quitin can also be attached to specific proteins. The consequences of most of these modifications are not yet understood. However, two recen t papers on one ubiquitin-like protein, SUMO-1, demonstrate a role in targeting a protein crucial for nucleocytoplasmic trafficking to the n uclear pore complex. These and other recent findings suggest a much wi der influence of the 'ubiquitin system' on cell biology and raise intr iguing regulatory and mechanistic questions.