Mitochondrial connection to the origin of the eukaryotic cell

Abstract

Phylogenetic evidence is presented that primitively amitochondriate eukaryotes containing the nucleus, cytoskeleton, and endomembrane system may have never existed. Instead, the primary host for the mitochondrial progenitor may have been a chimeric prokaryote, created by fusion between an archaebacterium and a eubacterium, in which eubacterial energy metabolism (glycolysis and fermentation) was retained. A Rickettsia-like intracellular symbiont, suggested to be the last common ancestor of the family Rickettsiaceae and mitochondria, may have penetrated such a host (pro-eukaryote), surrounded by a single membrane, due to tightly membrane-associated phospholipase activity, as do present-day rickettsiae. The relatively rapid evolutionary conversion of the invader into an organelle may have occurred in a safe milieu via numerous, often dramatic, changes involving both partners, which resulted in successful coupling of the host glycolysis and the symbiont respiration. Establishment of a potent energy-generating organelle made it possible, through rapid dramatic changes, to develop genuine eukaryotic elements. Such sequential, or converging, global events could fill the gap between prokaryotes and eukaryotes known as major evolutionary discontinuity.

Phosphoproteome Analysis of E. coli Reveals Evolutionary Conservation of Bacterial Ser/Thr/Tyr Phosphorylation

“…Traces of this evolutionary process can also be seen in the versatility of bacterial-specific kinase families, which evolved from nonkinase ancestors, such as the BY-kinases (15), the bifunctional Ser/Thr kinase/phosphorylases, and the tyrosine kinases with a guanidino-phosphotransferase domain (40). One important evolutionary factor that has also to be considered when examining the conservation of phosphoproteins is gene loss in parasites, which is mainly caused by the availability of the essential proteins from the host (41,42). An interesting example is Buchnera aphidicola, an intracellular symbiont of the greenbug aphid (43).…”

Apoptosis and the yeast actin cytoskeleton

“…As mitochondria are thought to have arisen from a bacterial progenitor, as endosymbionts of a primitive eukaryote, 25 it follows that divergent lineages may show common activity. An example of this exists within theintrinsic pathway of cell death that has been observed to contribute to programmed cell death in yeasts, 26 humans 27 and plants.…”

The reducible complexity of a mitochondrial molecular machine

“…Multiple lines of evidence show that the mitochondria in our cells evolved from intracellular bacteria (9)(10)(11), and that conversion of these intracellular bacteria into mitochondria required the evolution of protein transport machines (12). We proposed that simple ''core'' machines were established in the first eukaryotes by drawing on preexisting bacterial proteins that had previously provided distinct functions.…”

“…Protein transport into mitochondria requires the action of the 4 membrane-embedded molecular machines: the TOM, TIM22, TIM23, and SAM complexes (10)(11)(12)(13), each composed of up to 8 distinct protein subunits. However, bacteria do not import proteins across their outer and inner membranes, and the TOM and TIM23 complexes that provide this protein import function to mitochondria do not have counterparts in bacteria.…”

“…This protein transport machine has been studied in yeast, in plants, and in humans, and is composed of a conserved set of protein subunits that associate together to form the molecular machine (13)(14)(15). Three of the subunits are both found in enough representative groups to suggest they are present in all eukaryotic organisms, and are essential for cell viability in yeast: (i) the Tim23 subunit, a simple transmembrane protein that forms the channel through which protein substrates pass into the mitochondrial matrix; (ii) Tim44, found on the inner face of the mitochondrial membrane where it interacts with both Tim23 and Hsp70, thereby docking the protein import motor to the Tim23 channel; and (iii) the Tim14/ Pam18 subunit that interacts with several proteins in the TIM23 complex and directly stimulates the ATPase activity of Hsp70, thereby activating the motor to drive protein transport (10,(16)(17)(18). Here we show that ␣-proteobacteria have a protein of the Tim44 family that functions in membrane quality control and a Tim14/Pam18 protein that functions in a distinct process.…”