Mitochondrial function is critical for cells. These organelles are central for many cellular functions, including synthesizing ATP, steroid biosynthesis, fatty acid beta-oxidation and apoptosis. Drosophila mitochondria contain their own DNA, mtDNA, that encodes the same suite of products as human mtDNA. However, the vast majority of proteins used in the many biochemical pathways carried out by mitochondria are encoded in the nucleus and must be imported. Since respiration is a source of reactive species that can cause mtDNA damage, it is important for the cell to be able to maintain mitochondrial quality control. Because loss of mitochondrial function gives rise to specific mitochondrial diseases as well as is common in neurodegenerative disease, we are identifying proteins responsible for supporting mitochondrial function.
Towards this end, we have characterized the protein Clueless (Clu). Clu is a large, multi-domain protein that is highly conserved. We showed loss of Clu causes a drop in ATP, increased oxidative stress, short life lifespans, mislocalized mitochondria and sterility. Clu peripherally associates with mitochondria because it can form a complex with the Translocase of the Outer Mitochondrial membrane (TOM) 20, as well as Porin. In addition, we showed that Clu genetically and physically interacts with the PINK1-Parkin quality control mitophagy complex. These associations place Clu at the mitochondrial outer membrane and indicate that Clu’s affect on mitochondria is direct. Recently, we have found that Drosophila Clu is a ribonucleoprotein that binds mRNA, and that the tetratricopeptide repeat domain is largely responsible for this. In addition, Clu is able to bind both large and small ribosomal proteins, and does so at the mitochondrial outer membrane. Mitochondrial protein import can occur either post- or co-translationally, however the mechanisms controlling co-translational import have not been well characterized. Our data support a model by which Clu binds mRNAs, the ribosome, and the outer membrane translocase to facilitate co-translational import, which we are now testing.
How would Clu’s roles in protein import and mitochondrial quality control work together? We propose that Clu functions in protein import when mitochondria are healthy, but when the organelle is stressed and import no longer occurs, Clu contributes to PINK1-Park induced mitophagy in order to cull damaged mitochondria. By mediating both pathways, Clu may act as a sensor for mitochondrial quality.