Browsing by Person "Amin Fahim"

Now showing 1 - 2 of 2
  • Research Data
    Architectural principles of transporter-chaperone coupling within the native MHC I peptide-loading complex
    2025
    Milena Stolz
    Lukas Sušac
    Amin Fahim
    Rieke Keller
    Simon Trowitzsch
    Robert Tampé
    Adaptive immune responses are initiated by major histocompatibility class I (MHC I) presentation of antigenic peptides on the cell surface. This process relies on the peptide-loading complex (PLC), a dynamic transporter-multichaperone assembly in the endoplasmic reticulum (ER), to ensure high-fidelity selection, editing, and loading of peptides onto MHC I heterodimers. The PLC is the primary target for viral immune evasion, elicited in particular by human cytomegalovirus (HCMV), causing lifelong infections with severe risks for immunocompromised individuals. While the overall architecture of the PLC is known, how its activity is jeopardized by viral immune evasins remains unclear. Here, we present the 2.59–2.88 Å cryogenic electron microscopy structure of native human PLC associated with the HCMV immune evasin US6. US6 inhibits the heterodimeric transporter associated with antigen processing (TAP1/2) by latching its transmembrane helix laterally onto TAP2 and using its central disulfide-rich domain to mimic a translocating peptide. This effectively plugs the ER-lumenal exit and locks TAP in an outward-facing open conformation with closed nucleotide-binding domains and asymmetrically occluded ATP and ADP. In addition, the structure highlights the role of the unique N-terminal transmembrane domains of TAP as dynamic scaffolds that recruit the MHC I-specific chaperone tapasin by clamping its transmembrane helix to the core transmembrane domain of each transporter subunit. Our findings uncover how structural dynamics within human PLC are modulated by US6-mediated viral immune evasion and reveal potential targets for therapeutic modulation of antigen presentation in cancer and infectious diseases.
      32  3
  • Research Data
    Single-molecule dynamics reveals ATP binding alone powers substrate translocation by an ABC transporter
    2025-07-25
    Christoph Nocker
    Matija Pečak (DataCollector)
    Tobias Nocker (DataCollector)
    Amin Fahim (DataCollector)
    Lukas Susac (DataCollector)
    Robert Tampé (ContactPerson)
    ATP-binding cassette (ABC) transporters are molecular machines that are involved in a wide range of physiological processes, including antigen presentation by TAP, a key player in adaptive immunity. TAP and its bacterial homolog TmrAB use ATP to translocate peptides across membranes. To elucidate the mechanism of substrate transport, we employed a single-molecule FRET sensor to visualize single-translocation events by individual ABC transporters, overcoming the limitations of ensemble averaging. Using this approach, we tracked the membrane translocation of single peptides driven by a conformational switch from the inward- to the outward-facing state. In a slow-turnover TmrAB variant, even in the absence of Mg2+, we show that ATP binding alone is sufficient to drive peptide translocation. Structural analyses via cryogenic electron microscopy reveal that ATP binding induces an outward-facing conformation in both wild-type and mutant TmrAB, even without Mg2+. In wild-type TmrAB, this was sufficient for a single peptide translocation. However, Mg2+ was essential for the later stages of ATP hydrolysis and transporter resetting. Together, these results reveal a direct mechanistic link between ATP binding and substrate translocation at single-molecule resolution, providing new insights into the catalytic cycle of ABC transporters.
      23  5