A model for Scc2p stimulation of cohesin's ATPase and its inhibition by acetylation of Smc3p

Abstract

The evolutionarily conserved cohesin complex mediates sister chromatid cohesion and facilitates mitotic chromosome condensation, DNA repair, and transcription regulation. These biological functions require cohesin's two ATPases, formed by the Smc1p and Smc3p subunits. Cohesin's ATPase activity is stimulated by the Scc2p auxiliary factor. This stimulation is inhibited by Eco1p acetylation of Smc3p at an interface with Scc2p. It was unclear how cohesin's ATPase activity is stimulated by Scc2p or how acetylation inhibits Scc2p, given that the acetylation site is distal to cohesin's ATPase active sites. Here, we identify mutations in budding yeast that suppressed the in vivo defects caused by Smc3p acetyl-mimic and acetyl-defective mutations. We provide compelling evidence that Scc2p activation of cohesin ATPase depends on an interface between Scc2p and a region of Smc1p proximal to cohesin's Smc3p ATPase active site. Furthermore, substitutions at this interface increase or decrease ATPase activity to overcome ATPase modulation by acetyl-mimic and acetyl-null mutations. Using these observations and an existing cryo-EM structure, we propose a model for regulating cohesin ATPase activity. We suggest that Scc2p binding to Smc1p causes the adjacent Smc1p residues and ATP to shift, stimulating Smc3p's ATPase. This stimulatory shift is inhibited through acetylation of the distal Scc2p–Smc3p interface.

Chromosome organization by fine-tuning an ATPase

“…Finally, Boardman et al (2023) found further support for their model with the demonstration that the in vitro ATPase activity of Smc3-K113Q complexes cannot be stimulated by Scc2, but that Smc1-T1117I causes an increase in ATPase activity. In contrast, Smc1-T1117W displays lower activity.…”

“…Smc3 acetylation blocks loading and loop extrusion, thereby anchoring loops and stabilizing cohesion. Boardman et al (2023) provide genetic evidence that Scc2 modulates ATPase activity through interactions with Smc1 and that these interactions are blocked by Smc3 acetylation. Note that Scc3 is not shown for simplicity.…”

“…Since acetylation neutralizes the positive charges of lysine residues, it is reasonable to think that it would weaken the interaction of Scc2 with cohesin. Boardman et al (2023) build from this observation to investigate how acetylation affects cohesin interaction with Scc2 in vivo and collect a series of findings all supporting the hypothesis that acetylation directly decreases Scc2 interaction with SMC heads. First, they characterized the acetyl-mimicking smc3-K113Q mutation: Consistent with previous work, they found that it is not viable.…”

“…Cohesin's ATPase was known to be positively regulated by the Scc2 protein and negatively regulated by the acetylation of Smc3, but how these opposing effects function in vivo was not well understood. Boardman et al (2023) used budding yeast, combining suppressor screens, systematic analysis of point mutations, modeling using existing cohesin structures and functional assays in vivo to investigate the regulation of cohesin's ATPase activity. The findings lead to a molecular model that can explain how Scc2 and Smc3 acetylation influences cohesin's ATPase ( Fig.…”

“…Smc1-T1117 sits between residues potentially involved in the Smc1/Scc2 interaction and a residue (K1121) important for ATP binding. Boardman et al (2023) speculated that this position might be important for transmitting Scc2 binding to the ATPase site and promoting hydrolysis. If Scc2 binding is affected by acetylation or Smc3-K113Q, it might not be able to promote ATP hydrolysis.…”