Structural mechanism of ER retrieval of MHC class I by cowpox

Area of Science:

• Virology
• Immunology
• Structural Biology

Background:

• Viral immune evasion often involves disrupting major histocompatibility complex class I (MHCI) antigen presentation.
• Cowpox virus protein CPXV203 is known to interfere with MHCI surface expression via the KDEL-receptor pathway.

Purpose of the Study:

• To elucidate the mechanism by which CPXV203 interacts with and disrupts MHCI function.
• To characterize the structural and biochemical properties of the CPXV203/MHCI complex.

Main Methods:

• Biochemical binding assays to assess CPXV203 interaction with various MHCI proteins.
• pH-dependent stability studies of the CPXV203/MHCI complex.
• X-ray crystallography to determine the structure of the CPXV203/MHCI complex.
• Site-directed mutagenesis to identify key residues involved in the interaction.

Main Results:

• CPXV203 directly binds a broad range of assembled classical and non-classical MHCI proteins.
• The CPXV203/MHCI complex stability is highly pH-dependent, with increased affinity at lower pH (Golgi vs. ER).
• Crystallography revealed CPXV203 binds conserved MHCI sites under the peptide-binding platform, similar to other interacting proteins.
• Mutagenesis identified two histidine residues in CPXV203 crucial for low-pH stabilization and ER retrieval of MHCI.

Conclusions:

• CPXV203 employs a pH-dependent mechanism to bind MHCI, contributing to immune evasion.
• The identified binding interface and critical residues provide mechanistic insight into how CPXV203 disrupts antigen presentation.
• Understanding this interaction clarifies cowpox virus strategies for thwarting host T-cell detection.