Circular permutation in proteins

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Schematic representation of a circular permutation in two proteins. The first protein (outer circle) has the sequence a-b-c. After the permutation the second protein (inner circle) has the sequence c-a-b. The letters N and C indicate the location of the amino- and carboxy-termini of the protein sequences and how their positions change relative to each other. Circular Permutation In Proteins.svg
Schematic representation of a circular permutation in two proteins. The first protein (outer circle) has the sequence a-b-c. After the permutation the second protein (inner circle) has the sequence c-a-b. The letters N and C indicate the location of the amino- and carboxy-termini of the protein sequences and how their positions change relative to each other.

A circular permutation is a relationship between proteins whereby the proteins have a changed order of amino acids in their peptide sequence. The result is a protein structure with different connectivity, but overall similar three-dimensional (3D) shape. In 1979, the first pair of circularly permuted proteins – concanavalin A and lectin – were discovered; over 2000 such proteins are now known.

Contents

Circular permutation can occur as the result of evolutionary events, posttranslational modifications, or artificially engineered mutations. The two main models proposed to explain the evolution of circularly permuted proteins are permutation by duplication and fission and fusion. Permutation by duplication occurs when a gene undergoes duplication to form a tandem repeat, before redundant sections of the protein are removed; this relationship is found between saposin and swaposin. Fission and fusion occurs when partial proteins fuse to form a single polypeptide, such as in nicotinamide nucleotide transhydrogenases.

Circular permutations are routinely engineered in the laboratory to improve their catalytic activity or thermostability, or to investigate properties of the original protein.

Traditional algorithms for sequence alignment and structure alignment are not able to detect circular permutations between proteins. New non-linear approaches have been developed that overcome this and are able to detect topology-independent similarities.

History

Two proteins that are related by a circular permutation. Concanavalin A (left), from the Protein Data Bank (PDB: 3cna ), and peanut lectin (right), from PDB: 2pel , which is homologous to favin. The termini of the proteins are highlighted by blue and green spheres, and the sequence of residues is indicated by the gradient from blue (N-terminus) to green (C-terminus). The 3D fold of the two proteins is highly similar; however, the N- and C- termini are located on different positions of the protein. Concanavalin A vs Lectin.png
Two proteins that are related by a circular permutation. Concanavalin A (left), from the Protein Data Bank ( PDB: 3cna ​), and peanut lectin (right), from PDB: 2pel ​, which is homologous to favin. The termini of the proteins are highlighted by blue and green spheres, and the sequence of residues is indicated by the gradient from blue (N-terminus) to green (C-terminus). The 3D fold of the two proteins is highly similar; however, the N- and C- termini are located on different positions of the protein.

In 1979, Bruce Cunningham and his colleagues discovered the first instance of a circularly permuted protein in nature. [1] After determining the peptide sequence of the lectin protein favin, they noticed its similarity to a known protein – concanavalin A  – except that the ends were circularly permuted. Later work confirmed the circular permutation between the pair [2] and showed that concanavalin A is permuted post-translationally [3] through cleavage and an unusual protein ligation. [4]

After the discovery of a natural circularly permuted protein, researchers looked for a way to emulate this process. In 1983, David Goldenberg and Thomas Creighton were able to create a circularly permuted version of a protein by chemically ligating the termini to create a cyclic protein, then introducing new termini elsewhere using trypsin. [5] In 1989, Karolin Luger and her colleagues introduced a genetic method for making circular permutations by carefully fragmenting and ligating DNA. [6] This method allowed for permutations to be introduced at arbitrary sites. [6]

Despite the early discovery of post-translational circular permutations and the suggestion of a possible genetic mechanism for evolving circular permutants, it was not until 1995 that the first circularly permuted pair of genes were discovered. Saposins are a class of proteins involved in sphingolipid catabolism and antigen presentation of lipids in humans. Chris Ponting and Robert Russell identified a circularly permuted version of a saposin inserted into plant aspartic proteinase, which they nicknamed swaposin. [7] Saposin and swaposin were the first known case of two natural genes related by a circular permutation. [7]

Hundreds of examples of protein pairs related by a circular permutation were subsequently discovered in nature or produced in the laboratory. As of February 2012, the Circular Permutation Database [8] contains 2,238 circularly permuted protein pairs with known structures, and many more are known without structures. [9] The CyBase database collects proteins that are cyclic, some of which are permuted variants of cyclic wild-type proteins. [10] SISYPHUS is a database that contains a collection of hand-curated manual alignments of proteins with non-trivial relationships, several of which have circular permutations. [11]

Evolution

There are two main models that are currently being used to explain the evolution of circularly permuted proteins: permutation by duplication and fission and fusion. The two models have compelling examples supporting them, but the relative contribution of each model in evolution is still under debate. [12] Other, less common, mechanisms have been proposed, such as "cut and paste" [13] or "exon shuffling". [14]

Permutation by duplication

The permutation by duplication mechanism for producing a circular permutation. First, a gene 1-2-3 is duplicated to form 1-2-3-1-2-3. Next, a start codon is introduced before the first domain 2 and a stop codon after the second domain 1, removing redundant sections and resulting in a circularly permuted gene 2-3-1. Permutation by Duplication.svg
The permutation by duplication mechanism for producing a circular permutation. First, a gene 1-2-3 is duplicated to form 1-2-3-1-2-3. Next, a start codon is introduced before the first domain 2 and a stop codon after the second domain 1, removing redundant sections and resulting in a circularly permuted gene 2-3-1.

The earliest model proposed for the evolution of circular permutations is the permutation by duplication mechanism. [1] In this model, a precursor gene first undergoes a duplication and fusion to form a large tandem repeat. Next, start and stop codons are introduced at corresponding locations in the duplicated gene, removing redundant sections of the protein.

One surprising prediction of the permutation by duplication mechanism is that intermediate permutations can occur. For instance, the duplicated version of the protein should still be functional, since otherwise evolution would quickly select against such proteins. Likewise, partially duplicated intermediates where only one terminus was truncated should be functional. Such intermediates have been extensively documented in protein families such as DNA methyltransferases. [15]

Saposin and swaposin

Suggested relationship between saposin and swaposin. They could have evolved from a similar gene. Both consist of four alpha helices with the order of helices being permuted relative to each other. Saposin Swaposin.svg
Suggested relationship between saposin and swaposin. They could have evolved from a similar gene. Both consist of four alpha helices with the order of helices being permuted relative to each other.

An example for permutation by duplication is the relationship between saposin and swaposin. Saposins are highly conserved glycoproteins, approximately 80 amino acid residues long and forming a four alpha helical structure. They have a nearly identical placement of cysteine residues and glycosylation sites. The cDNA sequence that codes for saposin is called prosaposin. It is a precursor for four cleavage products, the saposins A, B, C, and D. The four saposin domains most likely arose from two tandem duplications of an ancestral gene. [16] This repeat suggests a mechanism for the evolution of the relationship with the plant-specific insert (PSI). The PSI is a domain exclusively found in plants, consisting of approximately 100 residues and found in plant aspartic proteases. [17] It belongs to the saposin-like protein family (SAPLIP) and has the N- and C- termini "swapped", such that the order of helices is 3-4-1-2 compared with saposin, thus leading to the name "swaposin". [7] [18]

Fission and fusion

The fission and fusion mechanism of circular permutation. Two separate genes arise (potentially from the fission of a single gene). If the genes fuse together in different orders in two orthologues, a circular permutation occurs. Fission-fusion (genetics).svg
The fission and fusion mechanism of circular permutation. Two separate genes arise (potentially from the fission of a single gene). If the genes fuse together in different orders in two orthologues, a circular permutation occurs.

Another model for the evolution of circular permutations is the fission and fusion model. The process starts with two partial proteins. These may represent two independent polypeptides (such as two parts of a heterodimer), or may have originally been halves of a single protein that underwent a fission event to become two polypeptides.

The two proteins can later fuse together to form a single polypeptide. Regardless of which protein comes first, this fusion protein may show similar function. Thus, if a fusion between two proteins occurs twice in evolution (either between paralogues within the same species or between orthologues in different species) but in a different order, the resulting fusion proteins will be related by a circular permutation.

Evidence for a particular protein having evolved by a fission and fusion mechanism can be provided by observing the halves of the permutation as independent polypeptides in related species, or by demonstrating experimentally that the two halves can function as separate polypeptides. [19]

Transhydrogenases

Transhydrogenases in various organisms can be found in three different domain arrangements. In cattle, the three domains are arranged sequentially. In the bacteria E. coli, Rb. capsulatus, and R. rubrum, the transhydrogenase consists of two or three subunits. Finally, transhydrogenase from the protist E. tenella consists of a single subunit that is circularly permuted relative to cattle transhydrogenase. Transhydrogenase Circular Permutations.svg
Transhydrogenases in various organisms can be found in three different domain arrangements. In cattle, the three domains are arranged sequentially. In the bacteria E. coli , Rb. capsulatus , and R. rubrum , the transhydrogenase consists of two or three subunits. Finally, transhydrogenase from the protist E. tenella consists of a single subunit that is circularly permuted relative to cattle transhydrogenase.

An example for the fission and fusion mechanism can be found in nicotinamide nucleotide transhydrogenases. [20] These are membrane-bound enzymes that catalyze the transfer of a hydride ion between NAD(H) and NADP(H) in a reaction that is coupled to transmembrane proton translocation. They consist of three major functional units (I, II, and III) that can be found in different arrangement in bacteria, protozoa, and higher eukaryotes. Phylogenetic analysis suggests that the three groups of domain arrangements were acquired and fused independently. [12]

Other processes that can lead to circular permutations

Post-translational modification

The two evolutionary models mentioned above describe ways in which genes may be circularly permuted, resulting in a circularly permuted mRNA after transcription. Proteins can also be circularly permuted via post-translational modification, without permuting the underlying gene. Circular permutations can happen spontaneously through autocatalysis, as in the case of concanavalin A. [4] Alternately, permutation may require restriction enzymes and ligases. [5]

Role in protein engineering

Many proteins have their termini located close together in 3D space. [21] [22] Because of this, it is often possible to design circular permutations of proteins. Today, circular permutations are generated routinely in the lab using standard genetics techniques. [6] Although some permutation sites prevent the protein from folding correctly, many permutants have been created with nearly identical structure and function to the original protein.

The motivation for creating a circular permutant of a protein can vary. Scientists may want to improve some property of the protein, such as:

Alternately, scientists may be interested in properties of the original protein, such as:

Algorithmic detection

Many sequence alignment and protein structure alignment algorithms have been developed assuming linear data representations and as such are not able to detect circular permutations between proteins. [34] Two examples of frequently used methods that have problems correctly aligning proteins related by circular permutation are dynamic programming and many hidden Markov models. [34] As an alternative to these, a number of algorithms are built on top of non-linear approaches and are able to detect topology-independent similarities, or employ modifications allowing them to circumvent the limitations of dynamic programming. [34] [35] The table below is a collection of such methods.

The algorithms are classified according to the type of input they require. Sequence-based algorithms require only the sequence of two proteins in order to create an alignment. [36] Sequence methods are generally fast and suitable for searching whole genomes for circularly permuted pairs of proteins. [36] Structure-based methods require 3D structures of both proteins being considered. [37] They are often slower than sequence-based methods, but are able to detect circular permutations between distantly related proteins with low sequence similarity. [37] Some structural methods are topology independent, meaning that they are also able to detect more complex rearrangements than circular permutation. [38]

NAMETypeDescriptionAuthorYearAvailabilityReference
FBPLOTSequenceDraws dot plots of suboptimal sequence alignmentsZuker1991 [39]
Bachar et al.Structure, topology independentUses geometric hashing for the topology independent comparison of proteinsBachar et al.1993 [35]
Uliel at alSequenceFirst suggestion of how a sequence comparison algorithm for the detection of circular permutations can workUliel et al.1999 [36]
SHEBAStructureUses SHEBA algorithm to create structural alignments for various permutation points, while iteratively improving the cut point.Jung & Lee2001 [14]
MultiprotStructure, Topology independentCalculates a sequence order independent multiple protein structure alignmentShatsky2004 server, download [38]
RASPODOMSequenceModified Needleman & Wunsch sequence comparison algorithm Weiner et al.2005 download [34]
CPSARSTStructureDescribes protein structures as one-dimensional text strings by using a Ramachandran sequential transformation (RST) algorithm. Detects circular permutations through a duplication of the sequence representation and "double filter-and-refine" strategy.Lo, Lyu2008 server [40]
GANGSTA +StructureWorks in two stages: Stage one identifies coarse alignments based on secondary structure elements. Stage two refines the alignment on residue level and extends into loop regions.Schmidt-Goenner et al.2009 server, download [41]
SANAStructureDetect initial aligned fragment pairs (AFPs). Build network of possible AFPs. Use random-mate algorithm to connect components to a graph.Wang et al.2010 download [42]
CE-CPStructureBuilt on top of the combinatorial extension algorithm. Duplicates atoms before alignment, truncates results after alignmentBliven et al.2015 server, download [43]
TopMatchStructureHas option to calculate topology-independent protein structure alignmentSippl & Wiederstein2012 server, download [44]

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<span class="mw-page-title-main">Protein</span> Biomolecule consisting of chains of amino acid residues

Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, responding to stimuli, providing structure to cells and organisms, and transporting molecules from one location to another. Proteins differ from one another primarily in their sequence of amino acids, which is dictated by the nucleotide sequence of their genes, and which usually results in protein folding into a specific 3D structure that determines its activity.

<span class="mw-page-title-main">Protein secondary structure</span> General three-dimensional form of local segments of proteins

Protein secondary structure is the local spatial conformation of the polypeptide backbone excluding the side chains. The two most common secondary structural elements are alpha helices and beta sheets, though beta turns and omega loops occur as well. Secondary structure elements typically spontaneously form as an intermediate before the protein folds into its three dimensional tertiary structure.

<span class="mw-page-title-main">Sequence alignment</span> Process in bioinformatics that identifies equivalent sites within molecular sequences

In bioinformatics, a sequence alignment is a way of arranging the sequences of DNA, RNA, or protein to identify regions of similarity that may be a consequence of functional, structural, or evolutionary relationships between the sequences. Aligned sequences of nucleotide or amino acid residues are typically represented as rows within a matrix. Gaps are inserted between the residues so that identical or similar characters are aligned in successive columns. Sequence alignments are also used for non-biological sequences such as calculating the distance cost between strings in a natural language, or to display financial data.

In bioinformatics, sequence analysis is the process of subjecting a DNA, RNA or peptide sequence to any of a wide range of analytical methods to understand its features, function, structure, or evolution. It can be performed on the entire genome, transcriptome or proteome of an organism, and can also involve only selected segments or regions, like tandem repeats and transposable elements. Methodologies used include sequence alignment, searches against biological databases, and others.

<span class="mw-page-title-main">Protein structure prediction</span> Type of biological prediction

Protein structure prediction is the inference of the three-dimensional structure of a protein from its amino acid sequence—that is, the prediction of its secondary and tertiary structure from primary structure. Structure prediction is different from the inverse problem of protein design. Protein structure prediction is one of the most important goals pursued by computational biology; it is important in medicine and biotechnology.

<span class="mw-page-title-main">Structural alignment</span> Aligning molecular sequences using sequence and structural information

Structural alignment attempts to establish homology between two or more polymer structures based on their shape and three-dimensional conformation. This process is usually applied to protein tertiary structures but can also be used for large RNA molecules. In contrast to simple structural superposition, where at least some equivalent residues of the two structures are known, structural alignment requires no a priori knowledge of equivalent positions. Structural alignment is a valuable tool for the comparison of proteins with low sequence similarity, where evolutionary relationships between proteins cannot be easily detected by standard sequence alignment techniques. Structural alignment can therefore be used to imply evolutionary relationships between proteins that share very little common sequence. However, caution should be used in using the results as evidence for shared evolutionary ancestry because of the possible confounding effects of convergent evolution by which multiple unrelated amino acid sequences converge on a common tertiary structure.

<span class="mw-page-title-main">Structural bioinformatics</span> Bioinformatics subfield

Structural bioinformatics is the branch of bioinformatics that is related to the analysis and prediction of the three-dimensional structure of biological macromolecules such as proteins, RNA, and DNA. It deals with generalizations about macromolecular 3D structures such as comparisons of overall folds and local motifs, principles of molecular folding, evolution, binding interactions, and structure/function relationships, working both from experimentally solved structures and from computational models. The term structural has the same meaning as in structural biology, and structural bioinformatics can be seen as a part of computational structural biology. The main objective of structural bioinformatics is the creation of new methods of analysing and manipulating biological macromolecular data in order to solve problems in biology and generate new knowledge.

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<span class="mw-page-title-main">Concanavalin A</span> Lectin (carbohydrate-binding protein) originally extracted from the jack-bean

Concanavalin A (ConA) is a lectin originally extracted from the jack-bean. It is a member of the legume lectin family. It binds specifically to certain structures found in various sugars, glycoproteins, and glycolipids, mainly internal and nonreducing terminal α-D-mannosyl and α-D-glucosyl groups. Its physiological function in plants, however, is still unknown. ConA is a plant mitogen, and is known for its ability to stimulate mouse T-cell subsets giving rise to four functionally distinct T cell populations, including precursors to regulatory T cells; a subset of human suppressor T-cells is also sensitive to ConA. ConA was the first lectin to be available on a commercial basis, and is widely used in biology and biochemistry to characterize glycoproteins and other sugar-containing entities on the surface of various cells. It is also used to purify glycosylated macromolecules in lectin affinity chromatography, as well as to study immune regulation by various immune cells.

<span class="mw-page-title-main">Fusion gene</span>

A fusion gene is a hybrid gene formed from two previously independent genes. It can occur as a result of translocation, interstitial deletion, or chromosomal inversion. Fusion genes have been found to be prevalent in all main types of human neoplasia. The identification of these fusion genes play a prominent role in being a diagnostic and prognostic marker.

<span class="mw-page-title-main">Transfer-messenger RNA</span>

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<span class="mw-page-title-main">TIM barrel</span> Protein fold

The TIM barrel, also known as an alpha/beta barrel, is a conserved protein fold consisting of eight alpha helices (α-helices) and eight parallel beta strands (β-strands) that alternate along the peptide backbone. The structure is named after triose-phosphate isomerase, a conserved metabolic enzyme. TIM barrels are ubiquitous, with approximately 10% of all enzymes adopting this fold. Further, five of seven enzyme commission (EC) enzyme classes include TIM barrel proteins. The TIM barrel fold is evolutionarily ancient, with many of its members possessing little similarity today, instead falling within the twilight zone of sequence similarity.

<span class="mw-page-title-main">Molecular biophysics</span> Interdisciplinary research area

Molecular biophysics is a rapidly evolving interdisciplinary area of research that combines concepts in physics, chemistry, engineering, mathematics and biology. It seeks to understand biomolecular systems and explain biological function in terms of molecular structure, structural organization, and dynamic behaviour at various levels of complexity. This discipline covers topics such as the measurement of molecular forces, molecular associations, allosteric interactions, Brownian motion, and cable theory. Additional areas of study can be found on Outline of Biophysics. The discipline has required development of specialized equipment and procedures capable of imaging and manipulating minute living structures, as well as novel experimental approaches.

<span class="mw-page-title-main">Protein domain</span> Self-stable region of a proteins chain that folds independently from the rest

In molecular biology, a protein domain is a region of a protein's polypeptide chain that is self-stabilizing and that folds independently from the rest. Each domain forms a compact folded three-dimensional structure. Many proteins consist of several domains, and a domain may appear in a variety of different proteins. Molecular evolution uses domains as building blocks and these may be recombined in different arrangements to create proteins with different functions. In general, domains vary in length from between about 50 amino acids up to 250 amino acids in length. The shortest domains, such as zinc fingers, are stabilized by metal ions or disulfide bridges. Domains often form functional units, such as the calcium-binding EF hand domain of calmodulin. Because they are independently stable, domains can be "swapped" by genetic engineering between one protein and another to make chimeric proteins.

Biological data visualization is a branch of bioinformatics concerned with the application of computer graphics, scientific visualization, and information visualization to different areas of the life sciences. This includes visualization of sequences, genomes, alignments, phylogenies, macromolecular structures, systems biology, microscopy, and magnetic resonance imaging data. Software tools used for visualizing biological data range from simple, standalone programs to complex, integrated systems.

Phyre and Phyre2 are free web-based services for protein structure prediction. Phyre is among the most popular methods for protein structure prediction having been cited over 1500 times. Like other remote homology recognition techniques, it is able to regularly generate reliable protein models when other widely used methods such as PSI-BLAST cannot. Phyre2 has been designed to ensure a user-friendly interface for users inexpert in protein structure prediction methods. Its development is funded by the Biotechnology and Biological Sciences Research Council.

<span class="mw-page-title-main">Plant-specific insert</span>

The plant-specific insert (PSI) or plant-specific sequence (PSS) is an independent domain, exclusively found in plants, consisting of approximately 100 residues, found on the C-terminal lobe on some aspartic proteases (AP) called phytepsins. The PSI, as an independent entity separate from its parent AP, is homologous to saposin and belongs to the saposin-like protein family (SAPLIP).

A protein superfamily is the largest grouping (clade) of proteins for which common ancestry can be inferred. Usually this common ancestry is inferred from structural alignment and mechanistic similarity, even if no sequence similarity is evident. Sequence homology can then be deduced even if not apparent. Superfamilies typically contain several protein families which show sequence similarity within each family. The term protein clan is commonly used for protease and glycosyl hydrolases superfamilies based on the MEROPS and CAZy classification systems.

<span class="mw-page-title-main">Protein tandem repeats</span>

An array of protein tandem repeats is defined as several adjacent copies having the same or similar sequence motifs. These periodic sequences are generated by internal duplications in both coding and non-coding genomic sequences. Repetitive units of protein tandem repeats are considerably diverse, ranging from the repetition of a single amino acid to domains of 100 or more residues.

References

Open Access logo PLoS transparent.svg This article was adapted from the following source under a CC BY 4.0 license (2012) (reviewer reports): Spencer Bliven; Andreas Prlić (2012). "Circular permutation in proteins". PLOS Computational Biology . 8 (3): e1002445. doi:10.1371/JOURNAL.PCBI.1002445. ISSN   1553-734X. PMC   3320104 . PMID   22496628. Wikidata   Q5121672.{{cite journal}}: CS1 maint: unflagged free DOI (link)

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