Analysis of the anti-apoptotic mechanisms of the molluscum contagiosum virus MC159 protein

Apoptosis is a type of cell death that is characterized by condensation of the nucleus and cytoplasm, DNA fragmentation and membrane blebbing. One function of apoptosis is as a defense mechanism used by the host to eliminate virus-infected cells. Viruses in turn have evolved their own mechanisms to interfere with host apoptotic signaling during infection to allow the virus time to reproduce. Molluscum contagiosum virus (MCV), a member of the human poxvirus family, produces small benign papules in the epidermis of the skin. MCV encodes the MC159 protein that interferes with the death receptor signaling pathways. The death effector domains (DED) of the MC159 protein are homologous to those of the cellular signaling molecules FADD and caspase-8. MC159 is thought to inhibit apoptosis by binding to FADD or caspase-8, and thereby interfering with the interaction between FADD and caspase-8.; To better understand the mechanism by which MC159 inhibits death receptor-induced apoptosis, mutational analysis of MC159 was used to determine the regions of MC159 that are important for its anti-apoptotic activity and for its interaction with other proteins. Binding of MC159 to FADD and caspase-8 was postulated to be sufficient to block apoptosis. Surprisingly, only three of the 19 mutants that lost the ability to protect against apoptosis were unable to bind FADD and caspase-8. Residues in the hydrophobic patch 1 within the predicted α2 region of either DED were required for binding to FADD and caspase-8 and to block apoptosis. An RXDL motif in the predicted α6 region of both MC159 DEDs, which is highly conserved among other DED-containing proteins, was necessary for MC159 to protect cells from Fas-, TNF-, and TRAIL-induced apoptosis and for MC159 to block formation of death effector filaments. The carboxy terminal region of MC159, after the second DED, was dispensable for its anti-apoptotic activity. While many other DED-containing proteins form oligomers, MC159 did not form self-oligomers.; The two DEDs of the anti-apoptotic MC159 were postulated to be interchangeable, while the DEDs of the pro-apoptotic caspase-8 were thought to be unable to substitute for those of MC159. Unexpectedly, the DEDs of MC159 were not functionally interchangeable and both DEDs in their correct order were needed to protect against apoptosis. The DEDs of caspase-8 were also unable to functionally substitute for the DEDs of MC159. These studies identify the critical regions of MC159 required to block apoptosis and to interact with cellular proteins.