Mortalin/GRP75 binds to complement C9 and plays a role in resistance to complement-dependent cytotoxicity

Area of Science:

• Cellular biology
• Immunology
• Molecular biology

Background:

• Mortalin/GRP75, a mitochondrial heat shock protein, is involved in cellular protection.
• Complement-dependent cytotoxicity (CDC) is a key immune defense mechanism.

Purpose of the Study:

• To investigate the role of mortalin in protecting cells against CDC.
• To determine the mechanism by which mortalin confers resistance to CDC.

Main Methods:

• K562 erythroleukemia cells were transfected to alter mortalin levels (synthesis interference or overexpression).
• Cellular localization of mortalin variants (mitochondrial vs. cytosolic) was assessed using GFP tagging.
• Binding of the C5b-9 membrane attack complex and complement proteins C8/C9 to cells was quantified.
• Purified mortalin domains (ATPase and substrate-binding) were tested for binding to complement proteins.

Main Results:

• Reduced mortalin synthesis increased sensitivity to CDC; mortalin overexpression conferred resistance.
• An inverse correlation was observed between C5b-9 deposition and cellular mortalin levels.
• Mitochondrial mortalin provided greater protection against CDC than cytosolic mortalin.
• The ATPase domain of mortalin, but not the substrate-binding domain, bound C8 and C9 and inhibited C9 polymerization.

Conclusions:

• Mitochondrial localization is crucial for mortalin's full protective effect against CDC.
• Mortalin inhibits CDC by targeting complement components C8 and C9 via its ATPase domain, thereby preventing C5b-9 assembly and stability.