Membrane Bound Enzyme Regulates Killer T-cells

Researchers have isolated and characterized a membrane bound enzyme called signal peptide peptidase (SPP) that plays a key role in regulating the activity of immune killer T cells. SSP was found to be a member of the family of proteolytic enzymes known as aspartic proteases. A paper published in the June 20, 2002, issue of Science reports that SSP modulates intramembrane proteolysis of signal peptides after they have been cleaved from molecules of the major histocompatibility complex I (MHC-I).

T cells "examine” the protein fragments on the cell surface, and if they recognize them, the T cells move on. If, however, the T cells do not recognize the fragments, the cell may be hosting a virus or manufacturing mutant proteins (as in the case of cancer). The T cells then react by attacking and killing the diseased cells. Recognition is due to the presence in the cell membrane of MHC-I fragments that have been generated by SSP proteolysis. If killer T cells detect insufficient levels of MHC-I protein fragments, they attack and destroy the suspect cell.

"Our research is a small part of the larger problem of how viruses and diseased cells ravage the body and circumvent our immune system, says Kathleen Binns, author of the paper and a doctoral student in medical genetics and microbiology at the University of Toronto (Canada). "This research gives us a better understanding of how the immune system works. As a result, we have a better understanding of how viruses and cancer try to get around this process. One day, we will hopefully be able to develop treatments and therapies to counter these rogue cells.”



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