Mechanism for Multiple Ligand Recognition by the Human Transferrin Receptor

Transferrin receptor 1 (TfR) plays a critical role in cellular iron import for most higher organisms. Cell surface TfR binds to circulating iron-loaded transferrin (Fe-Tf) and transports it to acidic endosomes, where low pH promotes iron to dissociate from transferrin (Tf) in a TfR-assisted process. The iron-free form of Tf (apo-Tf) remains bound to TfR and is recycled to the cell surface, where the complex dissociates upon exposure to the slightly basic pH of the blood. Fe-Tf competes for binding to TfR with HFE, the protein mutated in the iron-overload disease hereditary hemochromatosis. We used a quantitative surface plasmon resonance assay to determine the binding affinities of an extensive set of site-directed TfR mutants to HFE and Fe-Tf at pH 7.4 and to apo-Tf at pH 6.3. These results confirm the previous finding that Fe-Tf and HFE compete for the receptor by binding to an overlapping site on the TfR helical domain. Spatially distant mutations in the TfR protease-like domain affect binding of Fe-Tf, but not iron-loaded Tf C-lobe, apo-Tf, or HFE, and mutations at the edge of the TfR helical domain affect binding of apo-Tf, but not Fe-Tf or HFE. The binding data presented here reveal the binding footprints on TfR for Fe-Tf and apo-Tf. These data support a model in which the Tf C-lobe contacts the TfR helical domain and the Tf N-lobe contacts the base of the TfR protease-like domain. The differential effects of some TfR mutations on binding to Fe-Tf and apo-Tf suggest differences in the contact points between TfR and the two forms of Tf that could be caused by pH-dependent conformational changes in Tf, TfR, or both. From these data, we propose a structure-based model for the mechanism of TfR-assisted iron release from Fe-Tf

The 16th Data Release of the Sloan Digital Sky Surveys: First Release from the APOGEE-2 Southern Survey and Full Release of eBOSS Spectra

This paper documents the 16th data release (DR16) from the Sloan Digital Sky Surveys (SDSS), the fourth and penultimate from the fourth phase (SDSS-IV). This is the first release of data from the Southern Hemisphere survey of the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2); new data from APOGEE-2 North are also included. DR16 is also notable as the final data release for the main cosmological program of the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), and all raw and reduced spectra from that project are released here. DR16 also includes all the data from the Time Domain Spectroscopic Survey and new data from the SPectroscopic IDentification of ERosita Survey programs, both of which were co-observed on eBOSS plates. DR16 has no new data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey (or the MaNGA Stellar Library "MaStar"). We also preview future SDSS-V operations (due to start in 2020), and summarize plans for the final SDSS-IV data release (DR17)

The 16th Data Release of the Sloan Digital Sky Surveys: First Release from the APOGEE-2 Southern Survey and Full Release of eBOSS Spectra

This paper documents the 16th data release (DR16) from the Sloan Digital Sky Surveys (SDSS), the fourth and penultimate from the fourth phase (SDSS-IV). This is the first release of data from the Southern Hemisphere survey of the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2); new data from APOGEE-2 North are also included. DR16 is also notable as the final data release for the main cosmological program of the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), and all raw and reduced spectra from that project are released here. DR16 also includes all the data from the Time Domain Spectroscopic Survey and new data from the SPectroscopic IDentification of ERosita Survey programs, both of which were co-observed on eBOSS plates. DR16 has no new data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey (or the MaNGA Stellar Library "MaStar"). We also preview future SDSS-V operations (due to start in 2020), and summarize plans for the final SDSS-IV data release (DR17)

The 16th Data Release of the Sloan Digital Sky Surveys : First Release from the APOGEE-2 Southern Survey and Full Release of eBOSS Spectra

This paper documents the 16th data release (DR16) from the Sloan Digital Sky Surveys (SDSS), the fourth and penultimate from the fourth phase (SDSS-IV). This is the first release of data from the Southern Hemisphere survey of the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2); new data from APOGEE-2 North are also included. DR16 is also notable as the final data release for the main cosmological program of the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), and all raw and reduced spectra from that project are released here. DR16 also includes all the data from the Time Domain Spectroscopic Survey and new data from the SPectroscopic IDentification of ERosita Survey programs, both of which were co-observed on eBOSS plates. DR16 has no new data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey (or the MaNGA Stellar Library "MaStar"). We also preview future SDSS-V operations (due to start in 2020), and summarize plans for the final SDSS-IV data release (DR17).Peer reviewe

Biosensor Analyses of Fe-C-Lobe Binding to Immobilized Wild-Type and Selected Mutant TfR Molecules

<p>Plots of the equilibrium binding response, normalized to the R_max value (the ligand immobilization value) derived from fitting, versus concentration of injected Fe-C-lobe, are shown for the indicated TfR mutants along with the wild-type TfR control that was present in an adjacent flow cell on the same biosensor chip. Best-fit binding curves derived from a bivalent ligand model are shown as solid lines connecting the datapoints (squares for wild-type TfR and triangles for TfR mutants). The R651A mutant exhibited no binding and was not fit. A summary of derived binding constants is shown in the lower right panel. The K_Ds for wild-type TfR are averages derived from three independent measurements, and the number after the plus/minus sign represents the standard deviation.</p

Model for the Binding of Fe-Tf and Apo-Tf to TfR

<p>The figures representing each molecule are drawn to scale as an outline around the known structures of TfR (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0000051#pbio.0000051-Lawrence1" target="_blank">Lawrence et al. 1999</a>), Fe-ovo-Tf (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0000051#pbio.0000051-Kurokawa1" target="_blank">Kurokawa et al. 1995</a>), and apo-ovo-Tf (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0000051#pbio.0000051-Kurokawa2" target="_blank">Kurokawa et al. 1999</a>). Membrane-bound TfR includes a stalk region that places the TfR ectodomain about 30 Å above the cell surface (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0000051#pbio.0000051-Fuchs1" target="_blank">Fuchs et al. 1998</a>), which would allow the Tf molecule to extend below the plane of the TfR ectodomain. At basic pH, Fe-Tf (orange, with the iron atom positions shown as black dots) and TfR (blue) associate to make a complex containing one TfR homodimer and two Fe-Tf molecules, one bound to each polypeptide chain of the TfR homodimer. Fe-Tf makes energetically favorable contacts at basic pH to residues identified by mutagenesis in the TfR helical domain (red) and the protease-like domain (green). Acidification results in iron release and large conformational changes in the Tf structure as it becomes apo-Tf (gray). Apo-Tf does not make energetically favorable contacts with the protease-like domain, but retains binding to the helical domain-binding site (red) and makes new contacts to the helical domain (yellow), thereby stabilizing the complex. Upon return to basic pH, the apo-Tf molecules dissociate from TfR. This is also illustrated in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0000051#sv001" target="_blank">Video S1</a>.</p