SUMOylation and the predicted assembly of a plant NuRD-like repressor complex

TOPLESS and TOPLESS-RELATED protein are members of a conserved family of plant co-repressors. They act as general co-repressors to promote transcriptional repression. Loss of TOPLESS-mediated repression radically
affects development and other central processes. Repression via TOPLESS
is dependent on the activity of histone deacetylases, but direct interaction between the two has not been observed, although both TOPLESS and HISTONE DEACETYLASE 19 interact with proteins shared with the animal
Nucleosome Remodelling and histone Deacetylation complex. Some interactions between co-repressors and histone deacetylases are dependent
on the involvement of SMALL UBIQUITIN-LIKE MODIFIERS. TOPLESS and
TOPLESS–RELATED proteins are reported to be SUMOylated. The floral
phenotype of loss-of-function tpl-1 mutants is enhanced by the SUMO ligase mutant siz1-2. Abnormal tpl-1 flowers are not seen after crossing tpl-1 mutants with mutants of the SUMO proteases OVERLY TOLERANT TO
SALT 1 and 2. Mutations at predicted SUMOylation sites within TOPLESS
affect its ability to complement tpl-1 embryonic phenotypes and abolish
recovery of a high molecular-weight protein, potentially a TOPLESS-SUMO
conjugate. Attempts to co-precipitate SUMOylated TOPLESS and NuRD-associated proteins did not recover an intact complex but provided evidence to support heteromeric interactions between TOPLESS and TOPLESS-RELATED proteins. Additionally, the chromatin regulator SPLAYED was
identified as a potential interactor or antagonist. Development of novel
topless and topless-related mutant combinations shed light on the functional redundancy of family members and has created a platform for improved complementation assays. Additionally, discovery of an unreported root phenotype in the mutants will allow use of a more quantitative approach to phenotypic scoring. Phylogenetic analysis reveals the absence of a conserved subclade of the TOPLESS family from Arabidopsis with
implications for evolutionary and functional comparisons with other species, but suggests long-term conservation of a candidate SUMOylation site. SUMO may be involved in regulating TOPLESS function across plants.