The SIRT1 N-Terminal Domain as a Common Binding Interface for PPARγ Anchoring.

Insulin resistance, a global health threat linked to type 2 diabetes and obesity, can be addressed by modulating the activity of the Sirtuin 1 (SIRT1), a deacetylase that enhances insulin sensitivity by deacetylating the Peroxisome Proliferator-Activated Receptor Gamma (PPARγ) at lysine 268 and 293. Understanding the binding interfaces between SIRT1 and PPARγ is critical to developing new strategies to combat insulin resistance. In this study, we present four experimentally supported binding models of SIRT1 with acetylated PPARγ: one at position 268 and three at position 293 (SIRT1-_K268PPARγ and SIRT1-_K293PPARγ_1-3 models). These models were generated through an integration of in silico modeling and in vitro binding affinity assays. Our models revealed that the SIRT1:PPARγ binding interface is structured by SIRT1's 3-helix bundle in N-terminus domain (NTD(3HB)) and the catalytic domain (CD). The CD accommodated the acetylated peptide in its active site, while NTD(3HB) anchors PPARγ at a region between loops α1-β1 and α2'-α3 within PPARγ's ligand binding domain (LBD). Notably, the SIRT1-NTD(3HB) consistently bound to the same region of PPARγ in both models, highlighting a common mode for interaction. Through molecular dynamic simulation and binding assays, we demonstrated that either removal of SIRT1-NTD(3HB) or mutation within PPARγ-LBD significantly reduces binding affinity, underscoring the role of NTD(3HB) in substrate anchoring. Additionally, we provided evidence of SIRT1 dimerization, with substrate binding inducing its dissociation to form a heterodimer with PPARγ. These findings underscore the importance of the SIRT1 NTD(3HB) in PPARγ anchoring and offer insights into the activation mechanism of SIRT1, with potential implications for drug development targeting insulin resistance.