The nuclear dot protein Sp100, characterization of domains necessary for dimerization, subcellular localization, and modification by small ubiquitin-like modifiers

The Sp100 and promyelocytic leukemia proteins (PML) are constituents of nuc lear domains, known as nuclear dots (NDs) or PML bodies, and are both coval ently modified by the small ubiquitin-related protein SUMO-1. NDs play a ro le in autoimmunity, virus infections, and in the etiology of acute promyelo cytic leukemia. To date, little is known about the function of the Sp100 pr otein. Here we analyzed Sp100 domains that determine its subcellular locali zation, dimerization, and SUMOylation. A functional nuclear localization si gnal and an ND-targeting region that coincides with an Sp100 homodimerizati on domain were mapped. Sequences similar to the Sp100 homodimerization/ND-t argeting region occur in several other proteins and constitute a novel prot ein motif, termed HSR domain. The lysine residue of the Sp100 protein, to w hich SUMO-1 is covalently linked, was mapped within and may therefore modul ate the previously described HP1 protein-binding site. A consensus sequence for SUMOylation of proteins in general is suggested. SUMOylation strictly depended on a functional nuclear localization signal but was not necessary for nuclear import or ND targeting. A three-dimensional structure of Sp100, which supports the mapping data and provides additional information on Sp1 00 structure/function relationships, was generated by computer modeling. Ta ken together, our studies indicate the existence of well defined Sp100 doma ins with functions in ND targeting, nuclear import, nuclear SUMOylation, an d protein-protein interaction.