Novel human pro-apoptotic transmembrane proteins targeting the mitochondrial network – MITOMORT

MITOMORT is a basic science project focusing on the role of an endogenous interferon induced gene (ISG) encoding a small protein that targets the mitochondria leading to cell death. The title is a play on mitochondrion and the French word for death, mort. APOBEC3A is an ISG gene encoding a cytidine deaminase that is able to edit C residues in chromosomal DNA. The attack rate can be so high that it causes extensive double stranded breaks and apoptosis. This is referred to as hypermutation. Lower levels of mutation, hypomutation, occur and are associated with oncogenesis.

The Molecular Retrovirology Unit at the Pasteur Institute was the first to show that the APOBEC3A enzyme could attack chromosomal DNA. We noted that the initiation codons (AUG) of the two APOBEC3A isoforms used the “adequate” context according to the terminology of Marylin Kozak. Accordingly, we can expect that only 30-40% of ribosomes will settle on these sites, the remainder will continue to scan the mRNA. We asked the question, where will they settle? The next AUG downstream is in an “adequate” context while the following AUG is in a “strong” context. It turns out that initiation at these two sites produce two small proteins isoforms termes A3Ap3 and A3Ap4 (10.5 kDa and 8.6 kDa) that are in the same reading frame but overlapping that of APOBEC3A. They encode transmembrane spanning proteins that target the mitochondrion resulting in apoptosis.

We apparently have a unique situation where two pro-apoptotic proteins, APOBEC3A targeting the archive, the genome, and A3Ap3/A3Ap4 targeting the powerhouse, the mitochondrion, are encoded by a single gene – to date called APOBEC3A.

Research of A3Ap3/A3Ap4 apparently links apoptosis to the network of stress sensors that constitutes the interferon signalling pathway. It provides a link between the live cell and the death signal. Low levels of APOBEC3A will provide ongoing hypomutation and a weakening the mitochondrial network through sub-lethal doses of A3Ap3/A3Ap4. To compensate this the cell might slowly switch to ATP production via glycolysis as opposed to oxidative phosphorylation, or the Warburg effect.

The project seeks to understand the mechanism and biology of this small endogenous pro-apoptotic protein - the singular is used because, so far, the two isoforms appear to have exactly the same function.

The MITOMORT team combines the technology to resolve many facets associated with A3Ap3/A3Ap4, notably electron microscopy, confocal imagery and video. As intracellular obligate parasites have to protect them from premature apoptosis it is possible that nature has already developed an antagonist. To explore this hypothesis the consortium includes a laboratory with considerable experience in finding protein interactors, screening of a library of viral orfs as well microbial anti-apoptotic proteins. It is likely that A3Ap3/A3Ap4 is regulated leading to a fine balance between life and death. This would extend considerably the subject and provide new leads.

The two Pasteur labs have collaborated in the past while the lab in Tours is already collaborating with the MRU on the mitochondrial localization of A3Ap3/A3Ap4. The lab at Maison-Alfort is an obvious collaborating lab and is well known to the MRU even though they have never collaborated directly before.

While MITOMORT is a basic research project, as it concerns apoptosis mediated by an endogenous ISG, we feel that the findings will appeal to a wide audience of cell biologists. It provides a link between inflammation and cell death and so there is the possibility of it shedding some light on some autoimmune diseases like systemic lupus erythematosus. As to patents and the like, it is a little premature to make any predictions.