SUMO/sentrin: protein modifiers regulating important cellular functions

Regulation of protein functions can be achieved by posttranslational protei n modifications. One of the most studied modifications has been conjugation to ubiquitin, which mainly targets substrate proteins for degradation by t he 26 S proteasome. Recently, SUMO/sentrin, a ubiquitin-like protein has be en characterized. This evolutionary conserved protein is conjugated to spec ific proteins in a way similar, but not identical, to ubiquitin and seems a lso to be involved in the regulation of protein localization or function. A n increasing number of SUMO/sentrin substrates are currently described. We focus here on three major substrates of modification by SUMO: RanGAP1, PML, and I kappa B alpha proteins. These different examples illustrate how SUMO conjugation may be involved in the control of the level of critical protei ns within the cell or in the modulation of subcellular localization and nuc leocytoplasmic trafficking.