Fas is expressed in a number of lymphoma cell lines, on Epstein-Barr virus-transformed B lymphoblasts and on a proportion of activated B and T cells. Fas is also detected in soluble form and this form of the protein is thought to play a role in regulating certain aspects of immune system function. Elevated levels of soluble Fas is observed in leukemia and systemic lupus erythematosus.
Therefore, altered levels of secreted Fas protein is likely to be involved in the abnormal growth regulation of lymphoid cells.
Human CD95/Fas/Apo-1 antigen is a single transmembrane glycoprotein receptor of 325 amino acids (45-48 kDa) which activate cell apoptosis. The action of Fas is mediated via FADD (Fas-associated death domain)/ MORT1, an adapter protein that has a death domain at its C-terminus and binds to the cytoplasmic death domain of Fas.
APO-1/Fas(CD95) comprises of a death domain (DD) within the cytoplasmic region which triggers apoptosis upon binding of their cognate ligands. Once it is activated, APO-1/Fas(CD95) further aggregates its intracellular death domains which leads to the recruitment of two key signaling proteins followed by the formation of death-inducing signaling complex. These complex crosslinks through its C-terminal DD with APO-1/Fas receptors and engage caspase-8 via its N-terminal death effector domain (DED) to the DISC.