The binding of TCRs to their peptide-MHC ligands is characterized by a low affinity, slow kinetics, and a high degree of cross-reactivity. Here, we report the results of a kinetic and thermodynamic ...analysis of two TCRs binding to their peptide-MHC ligands, which reveal two striking features. First, significant activation energy barriers must be overcome during both association and dissociation, suggesting that conformational adjustments are required. Second, the low affinity of binding is a consequence of highly unfavorable entropic effects, indicative of a substantial reduction in disorder upon binding. This is evidence that the TCR and/or peptide-MHC have flexible binding surfaces that are stabilized upon binding. Such conformational flexibility, which may also be a feature of primary antibodies, is likely to contribute to cross-reactivity in antigen recognition.
The MHC class II molecule DQ0602 confers strong susceptibility to narcolepsy but dominant protection against type 1 diabetes. The crystal structure of DQ0602 reveals the molecular features underlying ...these contrasting genetic properties. Structural comparisons to homologous DQ molecules with differential disease associations highlight a previously unrecognized interplay between the volume of the P6 pocket and the specificity of the P9 pocket, which implies that presentation of an expanded peptide repertoire is critical for dominant protection against type 1 diabetes. In narcolepsy, the volume of the P4 pocket appears central to the susceptibility, suggesting that the presentation of a specific peptide population plays a major role.
The dimeric cell-surface glycoprotein CD8 is crucial to the positive selection of cytotoxic T cells in the thymus. The homodimer CD8alpha(alpha) or the heterodimer alpha beta stabilizes the ...interaction of the T-cell antigen receptor (TCR) with major histocompatibility complex (MHC) class I/peptide by binding to the class I molecule. Here we report the crystal structure at 2.7 A resolution of a complex between CD8alpha(alpha) and the human MHC molecule HLA-A2, which is associated with peptide. CD8alpha(alpha) binds one HLA-A2/peptide molecule, interfacing with the alpha2 and alpha3 domains of HLA-A2 and also contacting beta2-microglobulin. A flexible loop of the alpha3 domain (residues 223-229) is clamped between the complementarity-determining region (CDR)-like loops of the two CD8 subunits in the classic manner of an antibody-antigen interaction, precluding the binding of a second MHC molecule. The position of the alpha3 domain is different from that in uncomplexed HLA-A2, being most similar to that in the TCR/Tax/HLA-A2 complex, but no conformational change extends to the MHC/peptide surface presented for TCR recognition. Although these shifts in alpha3 may provide a synergistic modulation of affinity, the binding of CD8 to MHC is clearly consistent with an avidity-based contribution from CD8 to TCR-peptide-MHC interactions.
The enzyme BirA is a key reagent because of its ability to biotinylate proteins at a specific residue in a recognition sequence. We report a rapid, efficient, and economical method for the ...production, purification, and application of this enzyme. The method is easily scaled up and the protein produced is of high purity and can be stored for many months with retention of activity. We have used this enzyme to biotinylate the C termini of membrane proteins, allowing these proteins to be tetramerized by binding to streptavidin. Because of the specificity of the biotinylation at the C terminus, the orientation of the membrane proteins on the streptavidin is equivalent to that of the native protein on the cell surface. These tetrameric proteins can be used to study protein receptor–ligand interactions at the cell surface, and site-specific biotinylation can be used to study proteinsin vitrousing a defined orientation.
The yellow fever mosquito, Aedes aegypti, mates in flight as part of ephemeral aggregations termed swarms. Swarms contain many more males than females, and males are thought to be subject to intense ...sexual selection.1,2 However, which male traits are involved in mating success and the genetic basis of these traits remains unclear. We used an experimental evolution approach to measure genome-wide responses of Ae. aegypti evolved in the presence and absence of sexual selection. These data revealed for the first time how sexual selection shapes the genome of this important species. We found that populations evolved under sexual selection retained greater genetic similarity to the ancestral population and a higher effective population size than populations evolving without sexual selection. When we compared evolutionary regimes, we found that genes associated with chemosensation responded rapidly to the elimination of sexual selection. Knockdown of one high-confidence candidate gene identified in our analysis significantly decreased male insemination success, further suggesting that genes related to male sensory perception are under sexual selection. Several mosquito control technologies involve the release of males from captive populations into the wild. For these interventions to work, a released male must compete against wild males to successfully inseminate a female. Our results suggest that maintaining the intensity of sexual selection in captive populations used in mass-releases is important for sustaining both male competitive ability and overall genetic similarity to field populations.
•Sexual selection is important for retaining genetic similarity to field populations•Chemosensory genes evolve rapidly in response to the removal of sexual selection•Reduced expression of pickpocket315 affects male mating phenotype
Wyer et al. find that Aedes aegypti evolve rapidly in response to the elimination of sexual selection, with major genomic divergence in chemosensory genes. Populations evolving with sexual selection retain greater genetic similarity to their ancestors, highlighting the importance of maintaining sexual selection in captive populations.
The T cell surface glycoprotein CD8 enhances T cell antigen recognition by binding to MHC class I molecules. We show that human CD8 alpha alpha binds to the MHC class I molecule HLA-A2 with an ...extremely low affinity (Kd similar to 0.2 mM at 37 degree C) and with kinetics that are between 2 and 3 orders of magnitude faster than reported for T cell receptor/peptide-MHC interactions. Furthermore, CD8 alpha alpha had no detectable effect on a T cell receptor (TCR) binding to the same peptide-MHC class I complex. These binding properties provide an explanation as to why the CD8/MHC class I interaction is unable to initiate cell-cell adhesion and how it can enhance TCR recognition without interfering with its specificity.
The field of mosquito mating biology has experienced a considerable expansion in the past decade. Recent work has generated many key insights about specific aspects of mating behavior and physiology. ...Here, we synthesize these findings and classify swarming mosquito systems as polygynous. Male mating success is highly variable in swarms and evidence suggests that it is likely determined by both scramble competition between males and female choice. Incorporating this new understanding will improve both implementation and long-term stability of reproductive control tools.
The mating systems of medically important mosquitoes are characterized by aerial swarms, within which many complex behaviors unfold.Evidence suggests that females mate once, whereas males can mate multiply.This combined with swarms that consist of many more males than females generates intense mating competition between males and allows females to be choosy.A lack of data on male and female sexually selected traits and evolutionary relationships between them are a key knowledge gap in these systems.A comprehensive understanding of mosquito mating biology is essential for the development and successful deployment of reproductive control methods.
The cell surface molecules CD4 and CD8 greatly enhance the sensitivity of T-cell antigen recognition, acting as "co-receptors" by binding to the same major histocompatibility complex (MHC) molecules ...as the T-cell receptor (TCR). Here we use surface plasmon resonance to study the binding of CD8 alpha alpha to class I MHC molecules. CD8 alpha alpha bound the classical MHC molecules HLA- A*0201, -A*1101, -B*3501, and -C*0702 with dissociation constants (K sub(d)) of 90-220 mu M, a range of affinities distinctly lower than that of TCR/peptide-MHC interaction. We suggest such affinities apply to most CD8 alpha alpha /classical class I MHC interactions and may be optimal for T-cell recognition. In contrast, CD8 alpha alpha bound both HLA-A*6801 and B*4801 with a significantly lower affinity ( greater than or equal to 1 mM), consistent with the finding that interactions with these alleles are unable to mediate cell-cell adhesion. Interestingly, CD8 alpha alpha bound normally to the nonclassical MHC molecule HLA-G (K sub(d) similar to 150 mu M), but only weakly to the natural killer cell receptor ligand HLA-E (K sub(d) greater than or equal to 1 mM). Site-directed mutagenesis experiments revealed that variation in CD8 alpha alpha binding affinity can be explained by amino acid differences within the alpha 3 domain. Taken together with crystallographic studies, these results indicate that subtle conformational changes in the solvent exposed alpha 3 domain loop (residues 223-229) can account for the differential ability of both classical and nonclassical class I MHC molecules to bind CD8.
Public concern over the environmental and public health impacts of the emerging contaminant class "microplastics" has recently prompted government agencies to consider mitigation efforts. ...Microplastics do not easily fit within traditional risk-based regulatory frameworks because their persistence and extreme diversity (of size, shape, and chemical properties associated with sorbed chemicals) result in high levels of uncertainty in hazard and exposure estimates. Due to these serious complexities, addressing microplastics' impacts requires open collaboration between scientists, regulators, and policymakers. Here we describe ongoing international mitigation efforts, with California as a case study, and draw lessons from a similarly diverse and environmentally persistent class of emerging contaminants (per- and polyfluoroalkyl substances) that is already disrupting traditional regulatory paradigms, discuss strategies to address challenges associated with developing health-protective regulations and policies related to microplastics, and suggest ways to maximize impacts of research.
Characterisation of faecal indicator organism (FIO) concentrations and export coefficients for catchments with particular combinations of land use and under specific climatic regimes is critical in ...developing models to predict daily loads and apportion sources of the microbial parameters used to regulate water quality. Accordingly, this paper presents a synthesis of FIO concentration and export coefficient data for the summer bathing season, with some comparative winter data, for 205 river/stream sampling points widely distributed across mainland UK. In terms of both geometric mean (GM) FIO concentrations and export coefficients (expressed as cfukm−2h−1), the results reveal (1) statistically significant elevations at high flow compared with base flow, with concentrations typically increasing by more than an order of magnitude and export coefficients by about two orders; (2) significantly higher values in summer than in winter under high-flow conditions; and (3) extremely wide variability between the catchments (e.g. four orders of magnitude range for GM faecal coliform concentrations), which closely reflects land use—with urban areas and improved pastures identified as key FIO sources. Generally, these two most polluting land uses are concentrated in lowland areas where runoff (m3km−2h−1) is low compared with upland areas, which in the UK are dominated by rough grazing and forestry. Consequently, contrasts in export coefficients between land use types are less than for FIO concentrations. The GMs reported for most land use categories are based on ⩾13 sites and exhibit quite narrow confidence intervals. They may therefore be applied with some confidence to other catchments in the UK and similar geographical regions elsewhere. Examples are presented to illustrate how the results can be used to estimate daily summer base- and high-flow FIO loads for catchments with different land use types, and to assess the likely effectiveness of certain strategies for reducing FIO pollutant loadings in areas with extensive areas of lowland improved pasture.
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