Active Nuclear Receptors Exhibit Highly Correlated AF-2 Domain Motions

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Abstract
Nuclear receptor ligand binding domains (LBDs) convert ligand binding events into changes in gene expression by recruiting transcriptional coregulators to a conserved activation function-2 (AF-2) surface. While most nuclear receptor LBDs form homo- or heterodimers, the human nuclear receptor pregnane X receptor (PXR) forms a unique and essential homodimer and is proposed to assemble into a functional heterotetramer with the retinoid X receptor (RXR). How the homodimer interface, which is located 30 Å from the AF-2, would affect function at this critical surface has remained unclear. By using 20- to 30-ns molecular dynamics simulations on PXR in various oligomerization states, we observed a remarkably high degree of correlated motion in the PXR–RXR heterotetramer, most notably in the four helices that create the AF-2 domain. The function of such correlation may be to create “active-capable” receptor complexes that are ready to bind to transcriptional coactivators. Indeed, we found in additional simulations that active-capable receptor complexes involving other orphan or steroid nuclear receptors also exhibit highly correlated AF-2 domain motions. We further propose a mechanism for the transmission of long-range motions through the nuclear receptor LBD to the AF-2 surface. Taken together, our findings indicate that long-range motions within the LBD scaffold are critical to nuclear receptor function by promoting a mobile AF-2 state ready to bind coactivators. Long-range motions play essential roles in protein function but are difficult to appreciate from static crystal structures. We sought to understand how macromolecular motion affects the formation of transcriptional complexes central to controlling gene expression. Using 20- to 30-ns molecular dynamics simulations, we examined three nuclear receptors that function as ligand-regulated transcription factors: the pregnane X receptor, the peroxisome proliferator-activator receptor-γ, and estrogen receptor-α. We found that each of these receptors exhibits a high degree of correlated motions within the domain responsible for forming functionally essential protein–protein interactions with transcriptional coactivators. We further found that specific long-range (up to 30 Å) motions play an important role in these dynamics. Our results show that “active-capable” nuclear receptors are prepared for coactivator contacts by maintaining a mobile but preformed protein–protein interaction surface.

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