Camelid single-domain antibody fragments (‘nanobodies’) provide the remarkable specificity of antibodies within a single 15-kDa immunoglobulin V_HH domain. This unique feature has enabled applications ranging from use as biochemical tools to therapeutic agents. Nanobodies have emerged as especially useful tools in protein structural biology, facilitating studies of conformationally dynamic proteins such as G-protein-coupled receptors (GPCRs). Nearly all nanobodies available to date have been obtained by animal immunization, a bottleneck restricting many applications of this technology. To solve this problem, we report a fully in vitro platform for nanobody discovery based on yeast surface display. We provide a blueprint for identifying nanobodies, demonstrate the utility of the library by crystallizing a nanobody with its antigen, and most importantly, we utilize the platform to discover conformationally selective nanobodies to two distinct human GPCRs. To facilitate broad deployment of this platform, the library and associated protocols are freely available for nonprofit research.

The crystal structure of β-arrestin-1 in complex with a fully phosphorylated 29-amino-acid carboxy-terminal peptide derived from the V2 vasopressin receptor is reported; the structure of the complex shows striking conformational differences in β-arrestin-1 when compared with its inactive conformation. Arrestin proteins are negative regulators of G-protein-coupled receptor (GPCR) function and also act as G-protein-independent signalling proteins. Before forming a high-affinity complex, arrestins must be activated, and two papers in this issue of Nature focus on the interaction between GCPRs and activated arrestin at the atomic scale. Yong Ju Kim et al. mimicked the initial activation step by truncating the carboxy terminus of arrestin to produce the naturally occurring splice variant called p44 and determined its crystal structure. This structure provides insight into the role of naturally occurring truncated arrestins in the visual system. Arun Shukla et al. present the structure of non-visual β-arrestin-1 in complex with an antibody fragment (Fab30) and a fully phosphorylated 29-amino-acid C-terminal peptide derived from a GPCR, the arginine vasopressin type 2 receptor. Taken together, these two studies reveal striking conformational changes associated with arrestin activation. The functions of G-protein-coupled receptors (GPCRs) are primarily mediated and modulated by three families of proteins: the heterotrimeric G proteins, the G-protein-coupled receptor kinases (GRKs) and the arrestins1. G proteins mediate activation of second-messenger-generating enzymes and other effectors, GRKs phosphorylate activated receptors2, and arrestins subsequently bind phosphorylated receptors and cause receptor desensitization3. Arrestins activated by interaction with phosphorylated receptors can also mediate G-protein-independent signalling by serving as adaptors to link receptors to numerous signalling pathways4. Despite their central role in regulation and signalling of GPCRs, a structural understanding of β-arrestin activation and interaction with GPCRs is still lacking. Here we report the crystal structure of β-arrestin-1 (also called arrestin-2) in complex with a fully phosphorylated 29-amino-acid carboxy-terminal peptide derived from the human V2 vasopressin receptor (V2Rpp). This peptide has previously been shown to functionally and conformationally activate β-arrestin-1 (ref. 5). To capture this active conformation, we used a conformationally selective synthetic antibody fragment (Fab30) that recognizes the phosphopeptide-activated state of β-arrestin-1. The structure of the β-arrestin-1–V2Rpp–Fab30 complex shows marked conformational differences in β-arrestin-1 compared to its inactive conformation. These include rotation of the amino- and carboxy-terminal domains relative to each other, and a major reorientation of the ‘lariat loop’ implicated in maintaining the inactive state of β-arrestin-1. These results reveal, at high resolution, a receptor-interacting interface on β-arrestin, and they indicate a potentially general molecular mechanism for activation of these multifunctional signalling and regulatory proteins.