REVIEW
A Structure-Centric View of Protein Evolution, Design and Adaptation
Biomolecules · Submitted 22 Mar 2006 · 120 references
Pith's verdict: A useful review whose central anti-convergence claim overreaches: the oPDUG test only rejects uniform random subgraphs, not all state-independent convergent models.
T0 · AI review
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Referee report
The paper
The paper claims
Protein structures betray shared ancestry, not convergent sampling
The reading
What carries the argument
The load-bearing object is the Protein Domain Universe Graph (PDUG), whose nodes are protein domain families and whose edges connect domains whose structural similarity, measured by DALI Z-scores, exceeds a cutoff chosen at the critical point of the graph's giant-component transition. Each organism's structural proteome induces a subgraph called an oPDUG. The argument turns on comparing two observables: cluster-size distributions, which cannot distinguish evolved graphs from randomized ones, and degree distributions, especially the maximum degree $k_{\max}$, which can. Under the convergent null hypothesis an oPDUG should resemble a random subgraph of the PDUG; the paper shows it does not, and that a duplication-and-divergence model can reproduce the observed non-randomness.
Extended reading notes
Core claim
The paper's central claim is that protein structural evolution is dominated by divergence: new domain families arise from duplication and divergence of existing ones, and the observed network of structural similarity is the trace of that process rather than independent convergence on the same folds. The decisive observation is that organismal structural proteomes are highly non-random subgraphs of the Protein Domain Universe Graph. Comparing each oPDUG to random subgraphs of the PDUG with the same number of nodes, the authors found that the maximal connectivity ($k_{\max}$) in real proteomes is far larger than in random controls; for over two-thirds of the prokaryotic genomes considered, the probability that the oPDUG is a random subgraph falls below $10^{-6}$. This rules out "equilibrium convergence," meaning models in which the state of an organismal graph at time $t_0$ cannot influence which domains are sampled from the underlying graph at any later time $t$. A divergent Big Bang speciation model, by contrast, produces oPDUGs with the same low probabilities of being random subgraphs, and the authors show that no simple history-independent bias can rescue the convergent picture without producing unrealistic degree distributions.
Load-bearing premise
The case against convergence assumes that a convergent world would leave each organism with a random, history-independent sample of the protein-structure universe; if convergent evolution were instead biased by function or ecology, the random-subgraph test would not rule it out.
Editorial extensions
If this is right
- If the central claim is right, the scale-free degree distribution of the PDUG is a genuine evolutionary signal, not a trivial consequence of compact-polymer geometry.
- Fold-size distributions alone cannot arbitrate between evolutionary models, since randomized graphs reproduce them; topological observables such as degree distributions are required.
- Structural proteomes shaped by divergence provide legitimate phylogenetic characters, with Dollo parsimony as the tree-building method most consistent with the models.
- Designability and evolutionary history jointly determine family size: a structure's contact density sets its potential, while duplication and divergence determine how much of that potential is realized.
- Lateral transfer of whole structural domains should be rare enough that domain presence and absence track lineage history, which explains why domain-content phylogenies succeed.
Reading between the lines
- One extension the authors do not develop is applying the same random-subgraph test to other conserved genomic characters, such as functional modules or domain combinations, to ask whether the non-randomness is specific to structure or generic to descent.
- A testable extension would run the identical $k_{\max}$ and P-value calculation on eukaryotic oPDUGs as structural coverage improves; the paper's decisive dataset is mostly prokaryotic, and it is not obvious the conclusion carries over unchanged.
- The oPDUG argument leaves open convergence at the scale of local structural elements, such as active-site residue arrangements within globally divergent domains; that kind of convergence would not register as randomness in the graph.
Original abstract
Proteins, by virtue of their central role in most biological processes, represent one of the key subjects of the study of molecular evolution. Inherent to the indispensability of proteins for living cells is the fact that a given protein can adopt a specific three-dimensional shape that is specified solely by the proteins sequence of amino acids. Over the past several decades, structural biologists have demonstrated that the array of structures that proteins may adopt is quite astounding, and this has lead to a strong interest in understanding how protein structures change and evolve over time. In this review we consider a large body of recent work that attempts to illuminate this structure-centric picture of protein evolution. Much of this work has focused on the question of how completely new protein structures (i.e. new folds or topologies) are discovered by protein sequences as they evolve. Pursuant to this question of structural innovation has been a desire to describe and understand the observation that certain types of protein structures are far more abundant than others and how this uneven distribution of proteins implicates on the process through which new shapes are discovered. We consider a number of theoretical models that have been successful at explaining this heterogeneity in protein populations and discuss the increasing amount of evidence that indicates that the process of structural evolution involves the divergence of protein sequences and structures from one another.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
Proteins, by virtue of their central role in most biological processes, represent one of the key subjects of the study of molecular evolution. Inherent to the indispensability of proteins for living cells is the fact that a given protein can adopt a specific three-dimensional shape that is specified solely by the proteins sequence of amino acids. Over the past several decades, structural biologists have demonstrated that the array of structures that proteins may adopt is quite astounding, and this has lead to a strong interest in understanding how protein structures change and evolve over time. In this review we consider a large body of recent work that attempts to illuminate this structure-centric picture of protein evolution. Much of this work has focused on the question of how completely new protein structures (i.e. new folds or topologies) are discovered by protein sequences as they evolve. Pursuant to this question of structural innovation has been a desire to describe and understand the observation that certain types of protein structures are far more abundant than others and how this uneven distribution of proteins implicates on the process through which new shapes are discovered. We consider a number of theoretical models that have been successful at explaining this heterogeneity in protein populations and discuss the increasing amount of evidence that indicates that the process of structural evolution involves the divergence of protein sequences and structures from one another.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | Proteins, by virtue of their central role in most biological processes, represent one of the key subjects of the study of molecular evolution. | background 1.00 | 0.04 | 0.03 | asserted 1.00 |
| s1 | Inherent to the indispensability of proteins for living cells is the fact that a given protein can adopt a specific three-dimensional shape that is specified solely by the proteins sequence of amino acids. | background 1.00 | 0.04 | 0.03 | asserted 1.00 |
| s2 | Over the past several decades, structural biologists have demonstrated that the array of structures that proteins may adopt is quite astounding, and this has lead to a strong interest in understanding how protein structures change and evolve over time. | background 1.00 | 0.03 | 0.03 | asserted 0.99 |
| s3 | In this review we consider a large body of recent work that attempts to illuminate this structure-centric picture of protein evolution. | contribution 0.94 | 0.25 | 0.05 | asserted 0.79 |
| s4 | Much of this work has focused on the question of how completely new protein structures (i.e. new folds or topologies) are discovered by protein sequences as they evolve. | background 0.63 | 0.05 | 0.05 | asserted 0.94 |
| s5 | Pursuant to this question of structural innovation has been a desire to describe and understand the observation that certain types of protein structures are far more abundant than others and how this uneven distribution of proteins implicates on the process through which new shapes are discovered. | background 0.92 | 0.07 | 0.08 | asserted 0.91 |
| s6 | We consider a number of theoretical models that have been successful at explaining this heterogeneity in protein populations and discuss the increasing amount of evidence that indicates that the process of structural evolution involves the divergence of protein sequences and structures from one another. | contribution 0.81 | 0.15 | 0.29 | hedged possibility 0.85 |
What was examined, and what was not
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Figures
Showing 3 of the paper's 10 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:q-bio/0603028 by the authors.
References
Discussion
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