The membrane attack complex (MAC) of the complement system is causing membrane damage and cell death. For protection, cells have adopted several resistance mechanisms, including removal of the membrane-inserted MAC by vesiculation. To identify proteins involved in MAC vesiculation, extracellular proteins released from K562 cells in response to treatment with sub-lytic complement were separated by acrylamide gel electrophoresis and protein bands were extracted, digested into peptides and the peptides were analyzed by mass spectrometry. A 75-kDa protein that was abundant in the supernatant of complement-treated cells was identified as mortalin/GRP75. Analysis by western blotting demonstrated that as early as 5 min after exposure to sub-lytic doses of complement, mortalin was released from K562 cells. Mortalin was released after complete activation of the complement system and formation of C5b-8, and even more so when C5b-9 was formed. Other pore formers, such as streptolysin O and melittin, did not induce release of mortalin. As shown, mortalin can bind to complement C8 and C9 and is shed in vesicles containing C9 and complement MACs. Anti-mortalin antibodies reduced mortalin release from complement-treated cells and elevated the extent of cell death by complement. Inhibitors of protein kinase C and extracellular signal-regulated protein kinase also prevented mortalin release from complement-activated cells. These results suggest that mortalin/GRP75 promotes the shedding of membrane vesicles loaded with complement MAC and protects cells from complement-mediated lysis.
- Membrane attack complex
- Membrane vesiculation