G protein–coupled receptors (GPCRs) recruit β-arrestins to coordinate diverse cellular processes, but the structural dynamics driving this process are poorly understood. Atypical chemokine receptors (ACKRs) are intrinsically biased GPCRs that engage β-arrestins but not G proteins, making them a model system for investigating the structural basis of β-arrestin recruitment. Here, we performed nuclear magnetic resonance (NMR) experiments on 13 CH 3 -ε–methionine–labeled ACKR3, revealing that β-arrestin recruitment is associated with conformational exchange at key regions of the extracellular ligand-binding pocket and intracellular β-arrestin–coupling region. NMR studies of ACKR3 mutants defective in β-arrestin recruitment identified an allosteric hub in the receptor core that coordinates transitions among heterogeneously populated and selected conformational states. Our data suggest that conformational selection guides β-arrestin recruitment by tuning receptor dynamics at intracellular and extracellular regions.

The interaction of scavenger receptor BI (SR-BI) and high density lipoproteins (HDL) is the key step for the bodily removal of cholesterol. The HDL/SR-BI interaction is one of the concluding steps of the reverse cholesterol transport (RCT) pathway. In RCT, cholesterol within atherosclerotic plaques is taken up by HDL particles, which then dock on SR-BI at the surface of the liver, where cholesterol can be delivered converted into bile for excretion. Humans with mutations in the gene encoding SR-BI display elevated HDL-cholesterol levels and an increased risk of cardiovascular disease (CVD). Therefore, understanding the interaction between HDL and SR-BI is crucial to discovering ways to lower plasma cholesterol and modulate CVD risk. Due to the importance of SR-BI, the goal of these studies is to resolve a structure of functional full-length human SR-BI. We have expressed and purified full-length SR-BI using an insect cell Sf9 system. In order to assess SR-BI’s functions within this system, Sf9 cells were infected with baculovirus encoding empty vector or SR-BI, or left uninfected and plated into culture dishes. Expression of SR-BI significantly increased HDL cellular association and uptake of HDL-cholesteryl esters compared to empty vector and uninfected cells. Additionally, free cholesterol efflux was increased upon SR-BI expression. Lastly, SR-BI in Sf9 cells maintained its ability to form higher order oligomers in cells, which is crucial, as SR-BI oligomerization is important in cholesterol transport. Additionally, size exclusion chromatography shows that purified SR-BI is able to form oligomers in micelles. Lastly, using microscale thermophoresis, we demonstrated that purified SR-BI binds to its ligands (apolipoprotein A-I, holoparticle HDL, and oxidized LDL) with high affinity, regardless of the glycosylation state of SR-BI. Together, these assays represent the first steps in resolving a high-resolution structure of human full-length SR-BI and provide promise for delineating the ways in which the HDL/SR-BI relationship allows for efficient cholesterol clearance.

Both CD36 and SR‐B1 are glycosylated integral membrane proteins belonging to the class B scavenger receptor family and are known to play roles in cardiovascular and atherosclerotic disease. CD36 serves as the receptor for oxidized low density lipoprotein (oxLDL) and facilitates cholesterol accumulation in macrophages and development of atherosclerotic plaques. SR‐B1 (scavenger receptor class B type 1) is the primary receptor for high density lipoprotein (HDL), which removes cholesterol from the peripheral atherosclerotic plaques and delivers it to the liver for excretion. Current structural information about class B scavenger receptors is limited to only purified extracellular regions, transmembrane segments, or transient transfection studies in cultured cells, and remains a roadblock to studying the isolated functions of these lipid and cholesterol transporters. Recently, we have successfully expressed and purified full‐length human SR‐B1 and CD36 using an Sf9 insect cell/baculoviral infection system, which allows for the expression of glycosylated proteins. Purified full‐length receptors are a valuable addition to our scientific toolbox and allow us to ask novel questions about the structure‐function relationships of scavenger receptors. To verify function within Sf9 cell membranes, we expressed human full‐length constructs in plated Sf9 cells and observed significantly increased binding of DiI‐HDL and DiI‐oxLDL, as well as increased DiI‐lipid uptake when comparing SR‐B1 and CD36 to empty vector infected cells, respectively. When purified and solubilized in detergent micelles, both receptors remain stable over time as shown by Prometheus thermal shift assays. We have also demonstrated that purified proteins are able to bind to their native ligands (HDL to SR‐B1 and oxLDL to CD36) with high affinity by microscale thermophoresis (MST). However, as SR‐B1 and CD36 are both scavenger receptors, they are able to bind to a variety of lipoprotein ligands and we have demonstrated these interactions with varying binding affinities by MST. Glycosylation of both CD36 and SR‐B1 has been shown to be important for cell surface expression, but the role of glycosylation in ligand binding has remained uncharacterized. We are the first to demonstrate that the glycosylation status of CD36 and SR‐B1 does not alter binding affinities or protein stability. These purified receptors and functional assays provide promise for understanding the ways in which class B scavenger receptors perform their critical functions and lay the groundwork for future structure‐function studies.

High density lipoprotein (HDL) is considered anti‐atherogenic due to its ability to remove cholesterol from the periphery and deliver it to the liver via its receptor, scavenger receptor BI (SR‐BI). The interaction between HDL and SR‐BI is the most important mechanism to remove bodily cholesterol. SR‐BI is crucial for maintaining lipid homeostasis, as humans with SR‐BI mutations display impaired cholesterol clearance and an elevated risk of cardiovascular disease (CVD). As there currently exists no full‐length structure for any of the Class B scavenger receptors, the goal of this study is to resolve a high‐resolution structure of functional full‐length SR‐BI. This structure would be a pivotal addition to the scientific toolbox employed to uncover questions of cholesterol transport in the context of cardiovascular disease. As first steps in the purification process, suspended sf9 cells were infected with baculovirus containing human SR‐BI‐encoding bacmid. After 96 hours post‐infection, cells expressing SR‐BI were centrifuged, lysed, and membrane proteins were solubilized into mixed micelles. SR‐BI within micelles was then purified through a series of steps that included affinity column purification, desalting, and concentration. Upon successful purification, verifying the expression of functional SR‐BI at the cellular surface was a critical step. To assess SR‐BI function, sf9 cells were infected with baculovirus containing SR‐BI encoding bacmid or empty vector and plated. Expression of human full‐length SR‐BI significantly increased HDL cellular association and uptake of HDL‐cholesteryl esters compared to uninfected and empty vector infected cells. Additionally, free cholesterol efflux and membrane cholesterol distribution were increased with SR‐BI expression. Lastly, SR‐BI expressed in sf9 cells maintained its ability to form higher order oligomers, suggesting synthesized receptor displays its expected functions. This purification protocol and functional assays represent the first steps in resolving the first high‐resolution structure of human full‐length SR‐BI and provide promise for delineating the ways in which the HDL/SR‐BI relationship allows for efficient cholesterol clearance.