For perhaps 1.8 billion years after life first emerged on Earth, a sort of evolutionary writer's block stalled the development of organisms more complicated than single cells. Then, a burst of experimental creativity about 1.7 billion years ago brought the cell nucleus onto the scene, stashing the cell's genetic material inside a protective inner membrane and setting the stage for the evolution of more sophisticated creatures from yeast, say, to plants and human beings.
Now research shows that one of the most basic design principles of this new eukaryotic life-form - the gatekeeper to the cell nucleus known as the nuclear pore complex - is largely shared across the most distantly related eukaryotes. Its core components likely evolved once and for all and would be found in the nuclear pore complex of what is known as the last common eukaryotic ancestor.
The findings, by Rockefeller University researchers Brian T. Chait, Michael P. Rout and colleagues, add details to an unfolding picture of cellular evolution that shows a common architecture for the nuclear pore complex and the vehicles that evolved around the same time to transport cargoes between different parts of the cell, called coated vesicles. As early as 1980, Rockefeller professor Günter Blobel proposed that the internal membranes of cells - such as those encompassing the nucleus and vesicles - evolved from invaginations of the outer cell membrane. Rout and Chait suggested in 2004 that the nuclear pore complex and vesicle coats, which both contain α-solenoid and β-propeller protein folds, evolved from ancient molecules called protocoatomers that stabilized the membranes of these primordial internal structures.
"This work shows that the pore contains the signature of this ancient evolutionary event," says Rout, head of the Laboratory of Cellular and Structural Biology. "Some evolutionary biologists have argued that the resemblance is only superficial. This paper shows that that is not true. The resemblance is not skin deep. Indeed, it goes all the way to the core."
Published June 13 in Molecular & Cellular Proteomics, research performed by former graduate student Jeffrey A. DeGrasse, now at the United States Food and Drug Administration, picked apart the proteins that make up the nuclear pore complex of Trypanosoma brucei, a deadly, single-celled parasite responsible for African sleeping sickness. In the evolutionary tree of eukaryotic life, T. brucei is about as far removed from vertebrates like ourselves as possible, far more distant than commonly used model organisms such as yeast, fungi and plants. "The trypanosomes were at the wedding of the eukaryotes but were divorced the day after," says Rout, quoting his University of Cambridge colleague Mark C. Field, who also worked on the research. The idea is that if the nuclear pore complex proteins of trypanosomes are the same as those in vertebrates, then they almost certainly are also in the hypothetical cell known as the last common eukaryotic ancestor. "They are defining features of eukaryotes, just as warm blood plus hair form a defining feature of mammals," says Rout.