We live in a world of specialization, and intentional subdivision of labor is embedded into our beliefs. Within a typical day we might visit the dentist, buy milk at the grocery store, and prepare tomorrow’s lecture on electromagnetism for physics class. Moreover, each of these categories has an ordered group of subspecializations with parts neatly arranged and identified by their function. The dentist drills with power tools crafted for that purpose, the milk is a tiny part of an organized industry to feed us, and electromagnetism is incomprehensible without years of schooling. We accept specialized functions as a reality of...

Chapter 1 introduced the concept of gene sharing in broad strokes, emphasizing that a gene may encode a protein with more than one molecular function and that protein function(s) may change as a consequence of alterations in the cellular microenvironment. In short, the gene-sharing idea acknowledges that the many properties of proteins are exploited in various ways in a cellular context, and that proteins may shift molecular functions during evolution in ways that are not entirely dependent on changes in their amino acid sequence.

Relating a single structure to multiple functions and recognizing functional shifts are hardly new ideas in...

Genes have been defined by their absence when an organism manifests a recessive trait, by their damaging potential when mutated, and by their contributions to the extraordinary abilities of Olympic athletes and Nobel Laureates. Genes have been considered root causes for our talents and blamed for our limitations. They have been given independence by their ability to mutate randomly, leaving us at their mercy, and were even charged with a selfish life of their own. They have instilled fear as targets of eugenics and have raised hope by promises of gene therapy. Genes may have been redefined over time but...

The gene-sharing concept originated from investigations on the major water-soluble proteins—the crystallins—of the cellular eye lens (see Wistow and Piatigorsky,²²Piatigorsky and Wistow,²³and de Jong et al.²⁴for reviews). A typical vertebrate eye and a diagrammatic representation of a lens are shown in Figure 4.1. Crystallins contribute to the optical properties of the transparent lens optimizing refractive index for image formation on the retinal photoreceptors. For lack of better criteria, crystallins are defined simply as abundant water-soluble lens proteins because protein concentration affects refractive index. The connection between the concentration of a given protein and refractive index of...

Chapter 4 summarized gene sharing by the taxon-specific lens crystallins—abundant cytoplasmic proteins contributing to lens optics as well as having nonrefractive functions in other tissues. This chapter explores the intriguing parallel between a group of taxon-specific corneal-enriched proteins and the diverse lens crystallins.^454,454aThe abundance of these selected corneal proteins has led to their being called “corneal crystallins.” However, unlike the lens crystallins, the optical function(s) of the corneal crystallins are not established, making it premature to claim that they are multifunctional proteins engaged in gene sharing. The corneal crystallins are considered here because the combination of their taxon-specificity...

Crystallins are one example of proteins having different, tissue-specific functions or serving multiple roles in the same tissue. Review of the scientific literature makes it clear that protein multifunctionality is more the rule than the exception for protein behavior. Most, if not all, polypeptides are probably involved in multiple tasks, exploiting both similar and different regions to perform various functions.

Enzymes exemplify specialization. They show stereo and substrate specificity as they direct metabolism by their catalytic activity at precise positions in biochemical pathways. We do not think of enzymes as structural building blocks. Nonetheless, specific enzymes accumulate in the lens...

The previous chapters give examples of individual proteins with multiple functions. This chapter portrays how a particular cellular process—gene expression—calls upon existing components to create the complex, functional web that is used to transmit information from DNA to protein. I chose gene expression for this purpose in part due to its central role in biology. In addition, there are other reasons that make gene expression of interest with respect to gene sharing. The combinatorial use of existing proteins, rather than the addition of new proteins, has been recognized as a major contributor to the increasing complexity of species...

Phylogenetic studies at the molecular level have established that sequence alterations during evolution are correlated with functional shifts of homologous proteins.⁷⁵⁰The gene-sharing concept postulates that adoption of a new role does not necessarily require the abandonment of the old one and that the transition of functions may involve overlapping periods during which the ancestral and novel functions coexist. I discuss antifreeze proteins (AFPs) in this chapter because they represent examples of proteins (1) that have specialized entirely for a new function and lost the earlier function, (2) that have undergone a functional shift yet have retained the potential to...

Extensive research has established that gene duplication is intimately connected with the creation of new genes and with the innovation of protein functions during evolution. Early studies noted small intrachromosomal duplications inDrosophilaand claimed that duplicate genes are responsible for theBarphenotype in flies.⁸³⁵Later studies proposed that new genes are created by duplication^836,837—an hypothesis that was confirmed at the molecular level by showing that myoglobin and the α-, β-, and γ-hemoglobins are products of ancient gene duplications.^2,838

Ohno^3,839–841was a strong proponent of the idea that redundant genes are invaluable templates for innovating protein functions....

Earlier chapters of this book defined (Chapter 1), provided examples (Chapters 4–7), and considered the dynamism (Chapters 8, 9) of gene sharing as an evolutionary process. Historically genes have been subject to many definitions and in general these have not been unequivocally resolved by the molecular era (Chapter 3). In particular, ambiguities remain with respect to defining the structure and function of a “gene.” The previous chapters have focused mostly on the molecular biology of gene sharing by examining individual proteins, such as metabolic enzymes and various specific proteins commonly identified with one of their physiological roles (for example,...

Gene sharing concerns many aspects of biology: It is about the structure, expression, and nature of genes, about multiple functions of single polypeptides, and about existing metabolic pathways and evolutionary processes. Gene sharing contributes to sculpting new images from old forms; it is about molecular function but affects phenotypes resulting from complex processes, cellular behavior, and species diversity. The domain of gene sharing is the vast space between molecular events and systems biology. Mutations both in amino acid sequences and regulatory motifs affect gene sharing. Gene sharing can be analyzed by reductionism—the molecular details of gene expression and protein:...