Explosive advances in biological knowledge are often triggered by episodes of conceptual convergence—moments when scientists working on different experimental systems realize suddenly that they are studying the same mechanism. Knowledge explodes because lessons from one system quickly ignite sparks in the other systems. Notable examples include the discovery of introns in viruses and animal cells; the appreciation of GTP-binding proteins as central to vision, hormone action, cancer, and yeast mating; and the recognition of receptor-mediated endocytosis as essential for cellular nutrition, viral entry, and degradation of protein hormones.

In the field of cell signaling, the new millennium is witnessing a new conceptual convergence: the realization that transmembrane proteins can be cleaved within the plane of the membrane to liberate cytosolic fragments that enter the nucleus to control gene transcription. This mechanism, called regulated intramembrane proteolysis (Rip), influences processes as diverse as cellular differentiation, lipid metabolism, and the response to unfolded proteins. In addition to its presence in animal cells, Rip has been observed in bacteria, and, remarkably, the bacterial proteases are related evolutionarily to the ones used in animal cells. The past 3 months have witnessed no fewer than 12 papers describing Rip in cells from bacteria to humans (