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Signal transduction across cell membranes often requires assembly of heteromeric receptors with defined stoichiometry. In this issue of Structure, Morales-Perez et al. (2016) present elegant methods for the expression of heteromeric nicotinic acetylcholine receptors with a defined α4β2 stoichiometry involving controlled baculovirus-mediated transduction and subunit counting by measurement of two fluorescent...
The proteins Smu1 and RED have been jointly implicated in the regulation of alternative splicing, mitosis, and influenza virus infection, but how they interact and whether their diverse cellular functions are coupled is unknown. We identified an N-terminal region of Smu1 and a central region of RED that stably interact. Structural analyses revealed that the RED-binding region of Smu1 contains an N-terminal...
Rvb1 and Rvb2 are essential AAA+ proteins that interact together during the assembly and activity of diverse macromolecules including chromatin remodelers INO80 and SWR-C, and ribonucleoprotein complexes including telomerase and snoRNPs. ATP hydrolysis by Rvb1/2 is required for function; however, the mechanism that drives substrate remodeling is unknown. Here we determined the architecture of the...
Phosphatidylinositol 3,4,5-trisphosphate (PIP3)-dependent Rac exchanger 1 (P-Rex1) is a Rho guanine nucleotide exchange factor synergistically activated by PIP3 and Gβγ that plays an important role in the metastasis of breast, prostate, and skin cancer, making it an attractive therapeutic target. However, the molecular mechanisms behind P-Rex1 regulation are poorly understood. We determined structures...
Cyclic guanosine monophosphate (cGMP)-dependent protein kinase (PKG) is a key regulator of smooth muscle and vascular tone and represents an important drug target for treating hypertensive diseases and erectile dysfunction. Despite its importance, its activation mechanism is not fully understood. To understand the activation mechanism, we determined a 2.5 Å crystal structure of the PKG I regulatory...
Escherichia coli possesses a number of specific K+ influx and efflux systems that maintain an appropriate intracellular K+ concentration. Although regulatory mechanisms have been identified for a number of these transport systems, the exact mechanism through which K+ concentration is sensed in the cell remains unknown. In this work we show that Kbp (K+ binding protein, formerly YgaU), a soluble 16-kDa...
Talin plays an important role in regulating integrin-mediated signaling. Talin function is autoinhibited by intramolecular interactions between the integrin-binding F3 domain and the autoinhibitory domain (R9). We determined the crystal structure of a triple-domain fragment, R7R8R9, which contains R9 and the RIAM (Rap1-interacting adaptor molecule) binding domain (R8). The structure reveals a crystallographic...
Evidence supports a role of antibody-dependent cellular cytotoxicity (ADCC) toward transitional epitopes in the first and second constant (C1-C2) regions of gp120 (A32-like epitopes) in preventing HIV-1 infection and in vaccine-induced protection. Here, we describe the first successful attempt at isolating the inner domain (ID) of gp120 as an independent molecule that encapsulates the A32-like region...
The Lon AAA+ protease (LonA) plays important roles in protein homeostasis and regulation of diverse biological processes. LonA behaves as a homomeric hexamer in the presence of magnesium (Mg2+) and performs ATP-dependent proteolysis. However, it is also found that LonA can carry out Mg2+-dependent degradation of unfolded protein substrate in an ATP-independent manner. Here we show that in the presence...
The ability of oligomeric membrane proteins to assemble in different functional ratios of subunits is a common feature across many systems. Recombinant expression of hetero-oligomeric proteins with defined stoichiometries facilitates detailed structural and functional analyses, but remains a major challenge. Here we present two methods for overcoming this challenge: one for rapid virus titration and...
The selectivity filter of K+ channels contains four ion binding sites (S1–S4) and serves dual functions of discriminating K+ from Na+ and acting as a gate during C-type inactivation. C-type inactivation is modulated by ion binding to the selectivity filter sites, but the underlying mechanism is not known. Here we evaluate how the ion binding sites in the selectivity filter of the KcsA channel participate...
The rapidly growing volume of data being produced by next-generation sequencing initiatives is enabling more in-depth analyses of conservation than previously possible. Deep sequencing is uncovering disease loci and regions under selective constraint, despite the fact that intuitive biophysical reasons for such constraint are sometimes absent. Allostery may often provide the missing explanatory link...
Our recently solved high-resolution structure of the serotonin 5-HT3 receptor (5-HT3R) delivered the first detailed structural insights for a mammalian pentameric ligand-gated ion channel. Based on this structure, we here performed a total of 2.8-μs all-atom molecular dynamics simulations to unravel at atomic detail how neurotransmitter binding on the extracellular domain induces sequential conformational...
Here, we show that modern solution nuclear magnetic resonance (NMR) structures of RNA exhibit more steric clashes and conformational ambiguities than their crystallographic X-ray counterparts. To tackle these issues, we developed RNA-ff1, a new force field for structure calculation with Xplor-NIH. Using seven published NMR datasets, RNA-ff1 improves covalent geometry and MolProbity validation criteria...
The molybdenum cofactor (Moco) is essential for the catalytic activity of all molybdenum-containing enzymes with the exception of nitrogenase. Moco biosynthesis follows an evolutionarily highly conserved pathway and genetic deficiencies in the corresponding human enzymes result in Moco deficiency, which manifests itself in severe neurological symptoms and death in childhood. In humans the final steps...
COP1 proteins are E3 ubiquitin ligases that regulate phototropism in plants and target transcription factors for degradation in mammals. The substrate-binding region of COP1 resides within a WD40-repeat domain that also binds to Trib proteins, which are adaptors for C/EBPα degradation. Here we report structures of the human COP1 WD40 domain in isolation, and complexes of the human and Arabidopsis...
The Lon AAA+ protease (LonA) is an evolutionarily conserved protease that couples the ATPase cycle into motion to drive substrate translocation and degradation. A hallmark feature shared by AAA+ proteases is the stimulation of ATPase activity by substrates. Here we report the structure of LonA bound to three ADPs, revealing the first AAA+ protease assembly where the six protomers are arranged alternately...
SMYD3 is a lysine methyltransferase overexpressed in colorectal, breast, prostate, and hepatocellular tumors, and has been implicated as an oncogene in human malignancies. Methylation of MEKK2 by SMYD3 is important for regulation of the MEK/ERK pathway, suggesting the possibility of selectively targeting SMYD3 in RAS-driven cancers. Structural and kinetic characterization of SMYD3 was undertaken leading...
APH(2″)-Ia is a widely disseminated resistance factor frequently found in clinical isolates of Staphylococcus aureus and pathogenic enterococci, where it is constitutively expressed. APH(2″)-Ia confers high-level resistance to gentamicin and related aminoglycosides through phosphorylation of the antibiotic using guanosine triphosphate (GTP) as phosphate donor. We have determined crystal structures...
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