Oxygen content of transmembrane proteins over macroevolutionary time scales.

We observe that the time of appearance of cellular compartmentalization correlates with atmospheric oxygen concentration. To explore this correlation, we predict and characterize the topology of all transmembrane proteins in 19 taxa and correlate differences in topology with historical atmospheric oxygen concentrations. Here we show that transmembrane proteins, individually and as a group, were probably selectively excluding oxygen in ancient ancestral taxa, and that this constraint decreased over time when atmospheric oxygen levels rose. As this constraint decreased, the size and number of communication-related transmembrane proteins increased. We suggest the hypothesis that atmospheric oxygen concentrations affected the timing of the evolution of cellular compartmentalization by constraining the size of domains necessary for communication across membranes.