Talking Point
Moonlighting proteins

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Abstract

The idea of one gene—one protein—one function has become too simple because increasing numbers of proteins are found to have two or more different functions. The multiple functions of such moonlighting proteins add another dimension to cellular complexity and benefit cells in several ways. However, cells have had to develop sophisticated mechanisms for switching between the distinct functions of these proteins.

Section snippets

Mechanisms

The function of a moonlighting protein can vary as a consequence of changes in cellular localization, cell type, oligomeric state, or the cellular concentration of a ligand, substrate, cofactor or product (Fig. 1). These different mechanisms are not mutually exclusive and, in many cases, a protein uses a combination of methods to switch between functions.

Evolution of moonlighting proteins

The more extreme examples of moonlighting raise two questions. (1) How did moonlighting arise during evolution? (2) How does it benefit the cell? The simplest answer is that organisms evolve by making use of whatever is available. Several multifunctional proteins are ubiquitous enzymes—for example, the two glycolytic enzymes mentioned above. These glycolytic enzymes are present in eukaryotes, eubacteria and archea, and have probably been around for over a billion years—ample time for additional

Conclusions

The increasing number of proteins that are being found to moonlight adds another level to our understanding of cellular complexity. There is a great deal of interaction between macromolecules, and the modern cell is a sophisticated and highly organized network. Moonlighting proteins provide one way of coordinating cellular activities.

The number of moonlighting proteins also has implications for prediction of the functions and interactions of the many proteins whose sequences are becoming

Acknowledgements

I thank Greg Petsko for critical reading of the manuscript and helpful suggestions.

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