During the transformation of the bacterial symbiont into the mitochondrion, the organelle lost the capacity to secrete proteins. From the protein transport perspective, the free-living (A) and the endosymbiotic bacterium(B) was secreting proteins to the environment or to host cell cytosol, respectively. (C)Upon the transfer of the bacterial genes into host cell genome and because new functions were assigned to the organelle, proteins started to be imported from the host cell cytosol. (D) the protein export was thought to be lost from mitochondria (until now!). The export and import of proteins is depicted by blue and green arrows, respectively.
Mitochondrial Export Of Proteins - Peering Through A Keyhole Into The Early Days Of The Eukaryotic Cell.
The acquisition of the mitochondrion was probably the trigger for the entire eukaryogenesis. The engulfment and the integration of bacterial symbiont resulted into the formation of a new cellular compartment, which retained its double membrane and became the centre of cellular energy metabolism. Many of the ancestral traits of the bacterial ancestor were lost during the evolution of eukaryotes and our understanding of the molecular background beyond the original symbiotic relationship is thus far from complete. Our premise for the proposed project has been that bacteria readily secrete proteins and metabolites to the surrounding communities. Mitochondria were thought to be devoid of protein secretion pathways as these have not yet been identified in any cellular system. Recently, we have succeeded in identifying components of bacterial type 2 secretion system in mitochondria of several lineages of unicellular eukaryotes (protists). The phylogenetic distribution of the pathway suggests that the mitochondrion of the LECA was capable of secreting protein(s) to the cell cytosol. We endeavour to characterize the function of the secretion pathway and to identify proteins (and other biomolecules) secreted by protist mitochondria. Having in hand solid preliminary data, we believe that this project can reveal, on the molecular level, new aspects of the relationship between the (host) cell and the mitochondrion in the early days of the eukaryotic cell.