The human APOBEC3G (A3G) DNA cytosine deaminase restricts and hypermutates DNA-based parasites including HIV-1. The viral infectivity factor (Vif) prevents restriction by triggering A3G degradation. While the structure of the A3G catalytic domain is known, the structure of the N-terminal Vif-binding domain has proven more elusive. Here, evolution- and structure-guided mutagenesis was used to solubilize the Vif-binding domain of A3G permitting structural determination by NMR spectroscopy. A smaller zinc-coordinating pocket and altered helical packing distinguish it from catalytic domain structures, and help explain the reported inactivity of this domain. This soluble A3G N-terminal domain is bound by Vif, which enabled mutagenesis and biochemical experiments to identify a unique Vif-interacting surface formed by α1-β1, β2-α2, and β4-α4 loops. This structure sheds new light on the Vif-A3G interaction and provides critical information for future drug development.

Myasthenia gravis (MG) is a prototypical B cell-mediated autoimmune disease affecting 20–50 per 100,000 people. The majority of patients fall into two clinically distinguishable types based on whether they produce autoantibodies targeting the acetylcholine receptor (AChR-MG) or muscle specific kinase (MuSK-MG). The autoantibodies are pathogenic, but whether their generation is associated with broader defects in the B cell repertoire is unknown. To address this question, we performed deep sequencing of the B cell receptor repertoire of AChR-MG, MuSK-MG and healthy subjects to generate approximately 518,000 unique VH and VL sequences from sorted naïve and memory B cell populations. AChR-MG and MuSK-MG subjects displayed distinct gene segment usage biases in both VH and VL sequences within the naïve and memory compartments. The memory compartment of AChR-MG was further characterized by reduced positive selection of somatic mutations in the H-CDR regions and altered H-CDR3 physicochemical properties. The VL repertoire of MuSK-MG was specifically characterized by reduced V/J segment distance in recombined sequences, suggesting diminished VL receptor editing during B cell development. Our results identify large-scale abnormalities in both the naïve and memory B cell repertoires. Particular abnormalities were unique to either AChR-MG or MuSK-MG indicating that the repertoires reflect the distinct properties of the subtypes. These repertoire abnormalities are consistent with previously observed defects in B cell tolerance checkpoints in MG, thereby offering additional insight regarding the impact of tolerance defects on peripheral autoimmune repertoires. These collective findings point toward a deformed B cell repertoire as a fundamental component of MG.

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Holliday junction (HJ) resolvases are structure-specific endonucleases that cleave four-way DNA junctions (HJs) generated during DNA recombination and repair. Bacterial RuvC, a prototypical HJ resolvase, functions as homodimer and nicks DNA strands precisely across the junction point. To gain insights into the mechanisms underlying symmetrical strand cleavages by RuvC, we performed crystallographic and biochemical analyses of RuvC from Thermus thermophilus (T.th. RuvC). The crystal structure of T.th. RuvC shows an overall protein fold similar to that of Escherichia coli RuvC, but T.th. RuvC has a more tightly associated dimer interface possibly reflecting its thermostability. The binding mode of a HJ-DNA substrate can be inferred from the shape/charge complementarity between the T.th. RuvC dimer and HJ-DNA, as well as positions of sulfate ions bound on the protein surface. Unexpectedly, the structure of T.th. RuvC homodimer refined at 1.28 Å resolution shows distinct asymmetry near the dimer interface, in the region harboring catalytically important aromatic residues. The observation suggests that the T.th. RuvC homodimer interconverts between two asymmetric conformations, with alternating subunits switched on for DNA strand cleavage. This model provides a structural basis for the ‘nick-counter-nick’ mechanism in HJ resolution, a mode of HJ processing shared by prokaryotic and eukaryotic HJ resolvases.

The theoretical model of the polarization Coulomb field scattering (PCF) caused by the polarization charge density variation at the AlGaN/AlN interface in strained AlGaN/AlN/GaN heterostructure field-effect transistors has been developed. And the theoretical values for the electron drift mobility, which were calculated using the Matthiessen's rule that includes PCF, piezoelectric scattering, polar optical-phonon scattering, and interface roughness scattering, are in good agreement with our experimental values. Therefore, the theoretical model for PCF has been confirmed.