Properties and assembly mechanisms of ND10, PML bodies, or PODs

Nuclear domain 10 (ND10), also referred to as PML bodies or PODs, are discr ete interchromosomal accumulations of several proteins including PML and Sp 100. We describe here developments in the visualization of ND10 and the mec hanism of ND10 assembly made possible by the identification of proteins tha t are essential for this process using cell lines that lack individual ND10 -associated proteins. PML is critical for the proper Localization of all ot her ND10-associated proteins under physiological conditions. Introducing PM L into a PML -/- cell line by transient expression or fusion with PML-produ cing cells recruited ND10-associated proteins into de novo formed ND10, att esting to its essential nature in ND10 formation. This recruitment includes Daxx, a protein that can bind PML and is highly enriched in, condensed chr omatin in the absence of PML. The segregation of Daxx from condensed chroma tin to ND10 by increased accumulation of Sentrin/SUMO-1 modified PML sugges ts the presence of a variable equilibrium between these two nuclear sites. These findings identify the basic requirements for ND10 formation and sugge st a dynamic mechanism for protein recruitment to these nuclear domains con trolled by the SUMO-1 modification state of PML. Additional adapter protein s are suggested to exist by the behavior of Sp100, and Sp100 will provide t he basis for their identification. Further information about the dynamic ba lance of proteins between ND10 and the actual site of functional activity o f these proteins will establish whether ND10 function as homeostatic regula tors or only in storage of excess proteins destined for turnover. (C) 2000 Academic Press.