The molecular determinants of classical pathway complement inhibition by OspEF-related proteins of Borrelia...

Area of Science:

• Immunology
• Microbiology
• Structural Biology

Background:

• The complement system is crucial for innate immunity, with the classical pathway initiated by the C1 complex (C1r, C1s).
• Borrelia burgdorferi, the Lyme disease agent, employs complement evasion strategies.
• ElpB and ElpQ are outer surface lipoproteins from B. burgdorferi that inhibit classical pathway activation by targeting C1r and C1s.

Purpose of the Study:

• To elucidate the molecular mechanism by which Elp proteins inhibit classical pathway proteases.
• To identify the specific binding site and key residues on ElpQ involved in C1s interaction.

Main Methods:

• Hydrogen-deuterium exchange mass spectrometry (HDX-MS) was used to map C1s binding on ElpQ.
• Site-directed mutagenesis of ElpQ was guided by HDX-MS data.
• Surface plasmon resonance (SPR) was employed to assess binding affinities of ElpQ mutants to C1r and C1s.

Main Results:

• HDX-MS identified a putative C1s-binding site on ElpQ.
• Mutagenesis and SPR revealed that C-terminal residues of ElpQ, including a conserved tyrosine, are critical for C1r and C1s binding.
• This specific tyrosine residue is essential for the complement inhibitory function of both ElpQ and ElpB.

Conclusions:

• Elp proteins interact with classical pathway proteases C1r and C1s via specific molecular determinants.
• A conserved C-terminal tyrosine in ElpQ is vital for protease binding and subsequent complement inhibition.
• These findings enhance understanding of Borrelia burgdorferi's sophisticated complement evasion mechanisms.