Cooperation of Sumoylated Chromosomal Proteins in rDNA Maintenance

Abstract

SUMO is a posttranslational modifier that can modulate protein activities, interactions, and localizations. As the GFP-Smt3p fusion protein has a preference for subnucleolar localization, especially when deconjugation is impaired, the nucleolar role of SUMO can be the key to its biological functions. Using conditional triple SUMO E3 mutants, we show that defects in sumoylation impair rDNA maintenance, i.e., the rDNA segregation is defective and the rDNA copy number decreases in these mutants. Upon characterization of sumoylated proteins involved in rDNA maintenance, we established that Top1p and Top2p, which are sumoylated by Siz1p/Siz2p, most likely collaborate with substrates of Mms21p to maintain rDNA integrity. Cohesin and condensin subunits, which both play important roles in rDNA stability and structures, are potential substrates of Mms21, as their sumoylation depends on Mms21p, but not Siz1p and Siz2p. In addition, binding of cohesin and condensin to rDNA is altered in the mms21-CH E3-deficient mutant.

Author Summary

Disruption of the SUMO (small ubiquitin-like modifier) pathway by mutations is lethal in mammals and in budding yeast; however, the essential nature of its role remains unknown, mainly because only a small fraction of most substrate proteins is SUMO-modified. We argue that the clustering of SUMO modifications among subunits of multiprotein complexes or within biochemical pathways indicates that SUMO-modified fractions of target proteins may have specific cooperative activities, distinct from the functions of individual unmodified proteins. SUMO conjugation-mediated functions in nucleolar processes can potentially be examples of such specific cooperative pathways, as we show that SUMO conjugates have a strong preference for nucleolar localization in budding yeast. Moreover, we demonstrate that stable maintenance of the nucleolar DNA and nucleolus is dependent on the putative functional interaction between the sumoylation of topoisomerases I and II (by Siz1p/Siz2p) and substrates of Mms21p SUMO-conjugating activity.