Trypanosome brucei, the causative agent of African sleeping sickness, harbours a highly ordered, subpellicular microtubule cytoskeleton that defines many aspects of morphology, motility and virulence. This array of microtubules is associated with a large number of proteins involved in its regulation. Employing proximity-dependent biotinylation assay (BioID) using the well characterized cytoskeleton-associated protein CAP5.5 as a probe we identified CAP50 (Tb927.11.2610). This protein colocalizes with the subpellicular cytoskeleton microtubules but not with the flagellum. Depletion by RNAi results in defects in cytokinesis, morphology and partial disorganization of microtubule arrays. Published proteomics data indicate a possible association of CAP50 with two other, yet uncharacterized, cytoskeletal proteins, CAP52 (Tb927.6.5070) and CAP42 (Tb927.4.1300), which were therefore included in our analysis. We show that their depletion causes phenotypes similar to those described for CAP50 and that they are essential for cellular integrity.

Biotin-based proximity labeling approaches, such as BioID, have demonstrated their use for the study of mitochondria proteomes in living cells. The use of genetically engineered BioID cell lines enables the detailed characterization of poorly characterized processes such as mitochondrial co-translational import. In this process, translation is coupled to the translocation of the mitochon-drial proteins, alleviating the energy cost typically associated with the post-translational import relying on chaperone systems. However, the mechanisms are still unclear with only few actors identified but none that have been described in mammals yet. We thus profiled the TOM20 prox-isome using BioID, assuming that some of identified proteins could be molecular actors of the co-translational import in human cells. The obtained results showed a high enrichment of RNA bind-ing proteins close to the TOM complex. However, for the few selected candidates, we could not demonstrate a role in the mitochondrial co-translational import process. Nonetheless, we were able to demonstrate additional uses of our BioID cell line. Indeed, the experimental approach used in this study is thus proposed for the identification of mitochondrial co-translational import effec-tors and for the monitoring of protein entry inside mitochondria with a potential application in the prediction of mitochondrial protein half-life.

In the matrix of bacteria/mitochondria/chloroplasts, Lon acts as the degradation machine for soluble proteins. In stress periods, however, proteostasis and survival depend on the strongly conserved Clp/Hsp100 family. Currently, the targets of ATP-powered unfoldases/disaggregases ClpB and ClpX, and of peptidase ClpP heptameric rings, are still unclear. Trapping experiments and proteome profiling in multiple organisms triggered confusion, so we analyzed the consistency of ClpP-trap targets in bacteria. We also provide meta-analyses of protein interactions in humans, to elucidate where Clp family members are enriched. Furthermore, meta-analyses of mouse complexomics are provided. Genotype-phenotype correlations confirmed our concept. Trapping, proteome, and complexome data retrieved consistent coaccumulation of ClpXP with GFM1 and TUFM orthologs. ClpX shows broad interaction selectivity encompassing mitochondrial translation elongation, RNA granule, and nucleoid; ClpB preferentially attaches to mitochondrial RNA granule and translation initiation components; ClpP is enriched with them all, and associates with release/recycling factors. Mutations in ClpP cause Perrault syndrome, with phenotypes similar to defects in mtDNA/mtRNA. Thus, we propose that ClpB and ClpXP are crucial to counteract misfolded insoluble protein assemblies that contain nucleotide-phosphates. This insight is relevant to improve ClpP-modulating drugs that block bacterial growth, and for the treatment of human infertility, deafness and neurodegeneration.