Alternative catalytic residues in the active site of Esco acetyltransferases

Cohesin is a protein complex encircles the DNA and regulates the separation of sister chromatids during cell division. Following a catalytic mechanism that is insufficiently understood, Esco1 and Esco2 acetyltransferases acetylate Smc3 subunit of cohesin, thereby inducing a stabilization of cohesin on DNA. As a prerequisite for structure-guided investigation of enzymatic activity, we determine here the crystal structure of the mouse Esco2/CoA complex at 1.8 Å resolution. We reconstitute the entire cohesin as a tetrameric assembly and use it as a physiologically-relevant substrate for enzymatic assays in vitro . Furthermore, we employ cell-based complementation studies in mouse embryonic fibroblast deficient for Esco1 and Esco2 , as a means to identify catalytically-important residues in vivo . These analyses demonstrate that D567/S566 and E491/S527, located on opposite sides of the MmEsco2 active site cleft, are critical for catalysis. Our experiments supports a catalytic mechanism of acetylation where residues D567 and E491 are general bases that deprotonate the ε-amino group of lysine substrate, via two nearby serine residues - S566 and S527-that possess a proton relay function.