Although nearly half of today’s major pharmaceutical drugs target human integral membrane proteins (hIMPs), only 30 hIMP structures are currently available in the Protein Data Bank, largely owing to inefficiencies in protein production. Here we describe a strategy for the rapid structure determination of hIMPs, using solution NMR spectroscopy with systematically labeled proteins produced via cell-free expression. We report new backbone structures of six hIMPs, solved in only 18 months from 15 initial targets. Application of our protocols to an additional 135 hIMPs with molecular weight <30 kDa yielded 38 hIMPs suitable for structural characterization by solution NMR spectroscopy without additional optimization.

The precise knowledge of the subunit assembly process of NMDA receptors (NMDA-Rs) is essential to understand the receptor architecture and underlying mechanism of channel function. Because NMDA-Rs are obligatory heterotetramers requiring the GluN1 subunit, it is critical to investigate how GluN1 and GluN2 type subunits coassemble into tetramers. By combining approaches in cell biology, biochemistry, single particle electron microscopy, and x-ray crystallography, we report the mechanisms and phenotypes of mutant GluN1 subunits that are defective in receptor maturation. The T110A mutation in the N-terminal domain (NTD) of the GluN1 promotes heterodimerization between the NTDs of GluN1 and GluN2, whereas the Y109C mutation in the adjacent residue stabilizes the homodimer of the NTD of GluN1. The crystal structure of the NTD of GluN1 revealed the mechanism underlying the biochemical properties of these mutants. Effects of these mutations on the maturation of heteromeric NMDA-Rs were investigated using a receptor trafficking assay. Our results suggest that the NTDs of the GluN1 subunit initially form homodimers and the subsequent dimer dissociation is critical for forming heterotetrameric NMDA-Rs containing GluN2 subunits, defining a molecular determinant for receptor assembly. The domain arrangement of the dimeric NTD of GluN1 is unique among the ionotropic glutamate receptors and predicts that the structure and mechanism around the NTDs of NMDA-Rs are different from those of the homologous AMPA and kainate receptors.

CC chemokine ligand 14, CCL14, is a human CC chemokine that is of recent interest because of its natural ability, upon proteolytic processing of the first eight NH2-terminal residues, to bind to and signal through the human immunodeficiency virus type-1 (HIV-1) co-receptor, CC chemokine receptor 5 (CCR5). We report X-ray crystallographic structures of both full-length CCL14 and signaling-active, truncated CCL14 [9-74] determined at 2.23 and 1.8 A, respectively. Although CCL14 and CCL14 [9-74] differ in their ability to bind CCR5 for biological signaling, we find that the NH2-terminal eight amino acids (residues 1 through 8) are completely disordered in CCL14 and both show the identical mode of the dimeric assembly characteristic of the CC type chemokine structures. However, analytical ultracentrifugation studies reveal that the CCL14 is stable as a dimer at a concentration as low as 100 nM, whereas CCL14 [9-74] is fully monomeric at the same concentration. By the same method, the equilibrium between monomers of CCL14 [9-74] and higher order oligomers is estimated to be of EC1,4 = 4.98 microM for monomer-tetramer conversion. The relative instability of CCL14 [9-74] oligomers as compared to CCL14 is also reflected in the Kd's that are estimated by the surface plasmon resonance method to be approximately 9.84 and 667 nM for CCL14 and CCL14 [9-74], respectively. This approximately 60-fold difference in stability at a physiologically relevant concentration can potentially account for their different signaling ability. Functional data from the activity assays by intracellular calcium flux and inhibition of CCR5-mediated HIV-1 entry show that only CCL14 [9-74] is fully active at these near-physiological concentrations where CCL14 [9-74] is monomeric and CCL14 is dimeric. These results together suggest that the ability of CCL14 [9-74] to monomerize can play a role for cellular activation.