
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.