Recurring Features of SignalTransduction Pathways Reveal Evolutionary Relationships
II. Transducing and Storing Energy 15. Signal-Transduction Pathways: An Introduction to Information Metabolism 15.6. Recurring Features of Signal-Transduction Pathways Reveal Evolutionary Relationships Many features of signal-transduction pathways are ancient. For example, cAMP signals the need for energy in prokaryotes as well as eukaryotes, although the mechanisms for detecting cAMP are different. Similarly, the GTPbinding proteins the G subunits of the hetero-trimeric G proteins and the members of the Ras family are part of an α ancient superfamily of evolutionarily related proteins. Other members of this superfamily are proteins that cycle between ATP- and ADP-bound forms; these proteins function in ATP synthesis (Section 18.4.5) and in generating molecular motion (Chapter 34). The superfamily also includes proteins taking part in protein synthesis (Section 29.4.2). The key feature of these proteins is that they undergo significant conformational changes on binding nucleoside triphosphates and hydrolyzing them to nucleoside diphosphates. These proteins can thus function as molecular "on-off" switches. A domain with this ability must have arisen early in evolution and been adapted to meet a range of biochemical needs since. Other proteins crucial to signal-transduction pathways arose much later. For example, the eukaryotic protein kinases are one of the largest protein families in all eukaryotes and yet appear to be absent in prokaryotes. The evolution of the eukaryotic protein kinase domain appears to have been an important biochemical step in the appearance of eukaryotes and the subsequent development of multicellular organisms. Entire signaling pathways have been conserved between organisms. For example, a key pathway in eye development in Drosophila is completely analogous to the EGF pathways in human beings (Figure 15.38). Thus, the wiring of this growth-control pathway is at least 800 million years old. II. Transducing and Storing Energy 15. Signal-Transduction Pathways: An Introduction to Information Metabolism 15.6. Recurring Features of Signal-Transduction Pathways Reveal Evolutionary Relationships Figure 15.38. Pathway Conservation. A pathway homologous to the mammalian EGF signal-transduction pathway functions in Drosophila to control the development of a specific photoreceptor cell in the eye.