Eukaryotic protein domains as functional units of cellular evolution

Interaction domains of Sos1/Grb2 are finely tuned for cooperative control of embryonic stem cell fate

Metazoan evolution involves increasing protein domain complexity, but how this relates to control of biological decisions remains uncertain. The Ras guanine nucleotide exchange factor (RasGEF) Sos1 and its adaptor Grb2 are multidomain proteins that couple fibroblast growth factor (FGF) signaling to activation of the Ras-Erk pathway during mammalian development and drive embryonic stem cells toward the primitive endoderm (PrE) lineage. We show that the ability of Sos1/Grb2 to appropriately regulate pluripotency and differentiation factors and to initiate PrE development requires collective binding of multiple Sos1/Grb2 domains to their protein and phospholipid ligands. This provides a cooperative system that only allows lineage commitment when all ligand-binding domains are occupied. Furthermore, our results indicate that the interaction domains of Sos1 and Grb2 have evolved so as to bind ligands not with maximal strength but with specificities and affinities that maintain cooperativity. This optimized system ensures that PrE lineage commitment occurs in a timely and selective manner during embryogenesis.

Systematic characterization of the specificity of the SH2 domains of cytoplasmic tyrosine kinases

Cytoplasmic tyrosine kinases (CTK) generally contain a Src-homology 2 (SH2) domain, whose role in the CTK family is not fully understood. Here we report the determination of the specificity of 25 CTK SH2 domains by screening one-bead-one-compound (OBOC) peptide libraries. Based on the peptide sequences selected by the SH2 domains, we built Support Vector Machine (SVM) models for the prediction of binding ligands for the SH2 domains. These models yielded support for the progressive phosphorylation model for CTKs in which the overlapping specificity of the CTK SH2 and kinase domains has been proposed to facilitate targeting of the CTK substrates with at least two potential phosphotyrosine (pTyr) sites. We curated 93 CTK substrates with at least two pTyr sites catalyzed by the same CTK, and showed that 71% of these substrates had at least two pTyr sites predicted to bind a common CTK SH2 domain. More importantly, we found 34 instances where there was at least one pTyr site predicted to be recognized by the SH2 domain of the same CTK, suggesting that the SH2 and kinase domains of the CTKs may cooperate to achieve progressive phosphorylation of a protein substrate. This article is part of a Special Issue entitled: From protein structures to clinical applications.

Dynamics and adaptive benefits of modular protein evolution

During protein evolution, novel domain arrangements are continuously formed. Rearrangements are important for the creation of molecular biodiversity and for functional molecular changes which underlie developmental shifts in the bauplan of organisms. Here we review the mechanisms by which new arrangements arise and the potential benefits of rearrangements. We concentrate on how new domains emerge and why they rapidly spread across genomes, gaining higher copy numbers than older, more established domains. This spread is most likely a consequence of their high adaptive potential but is unlikely to make up on its own for the drastic loss of domains, which is observed across different taxa. We show that a significant portion of the recently emerged domains, especially those in multidomain families, are highly disordered and speculate about the significance of these findings for the evolvability of novel genetic material.

Human 14-3-3 paralogs differences uncovered by cross-talk of phosphorylation and lysine acetylation

The 14-3-3 protein family interacts with more than 700 different proteins in mammals, in part as a result of its specific phospho-serine/phospho-threonine binding activity. Upon binding to 14-3-3, the stability, subcellular localization and/or catalytic activity of the ligands are modified. Seven paralogs are strictly conserved in mammalian species. Although initially thought as redundant, the number of studies showing specialization is growing. We created a protein-protein interaction network for 14-3-3, kinases and their substrates signaling in human cells. We included information of phosphorylation, acetylation and other PTM sites, obtaining a complete representation of the 14-3-3 binding partners and their modifications. Using a computational system approach we found that networks of each 14-3-3 isoform are statistically different. It was remarkable to find that Tyr was the most phosphorylatable amino acid in domains of 14-3-3 epsilon partners. This, together with the over-representation of SH3 and Tyr_Kinase domains, suggest that epsilon could be involved in growth factors receptors signaling pathways particularly. We also found that within zeta's network, the number of acetylated partners (and the number of modify lysines) is significantly higher compared with each of the other isoforms. Our results imply previously unreported hidden differences of the 14-3-3 isoforms interaction networks. The phosphoproteome and lysine acetylome within each network revealed post-transcriptional regulation intertwining phosphorylation and lysine acetylation. A global understanding of these networks will contribute to predict what could occur when regulatory circuits become dysfunctional or are modified in response to external stimuli.

Quantification and functional analysis of modular protein evolution in a dense phylogenetic tree

Modularity is a hallmark of molecular evolution. Whether considering gene regulation, the components of metabolic pathways or signaling cascades, the ability to reuse autonomous modules in different molecular contexts can expedite evolutionary innovation. Similarly, protein domains are the modules of proteins, and modular domain rearrangements can create diversity with seemingly few operations in turn allowing for swift changes to an organism's functional repertoire. Here, we assess the patterns and functional effects of modular rearrangements at high resolution. Using a well resolved and diverse group of pancrustaceans, we illustrate arrangement diversity within closely related organisms, estimate arrangement turnover frequency and establish, for the first time, branch-specific rate estimates for fusion, fission, domain addition and terminal loss. Our results show that roughly 16 new arrangements arise per million years and that between 64% and 81% of these can be explained by simple, single-step modular rearrangement events. We find evidence that the frequencies of fission and terminal deletion events increase over time, and that modular rearrangements impact all levels of the cellular signaling apparatus and thus may have strong adaptive potential. Novel arrangements that cannot be explained by simple modular rearrangements contain a significant amount of repeat domains that occur in complex patterns which we term "supra-repeats". Furthermore, these arrangements are significantly longer than those with a single-step rearrangement solution, suggesting that such arrangements may result from multi-step events. In summary, our analysis provides an integrated view and initial quantification of the patterns and functional impact of modular protein evolution in a well resolved phylogenetic tree. This article is part of a Special Issue entitled: The emerging dynamic view of proteins: Protein plasticity in allostery, evolution and self-assembly.

SH3 interactome conserves general function over specific form

Src homology 3 (SH3) domains bind peptides to mediate protein-protein interactions that assemble and regulate dynamic biological processes. We surveyed the repertoire of SH3 binding specificity using peptide phage display in a metazoan, the worm Caenorhabditis elegans, and discovered that it structurally mirrors that of the budding yeast Saccharomyces cerevisiae. We then mapped the worm SH3 interactome using stringent yeast two-hybrid and compared it with the equivalent map for yeast. We found that the worm SH3 interactome resembles the analogous yeast network because it is significantly enriched for proteins with roles in endocytosis. Nevertheless, orthologous SH3 domain-mediated interactions are highly rewired. Our results suggest a model of network evolution where general function of the SH3 domain network is conserved over its specific form.

Evolution of SH2 domains and phosphotyrosine signalling networks

Src homology 2 (SH2) domains mediate selective protein-protein interactions with tyrosine phosphorylated proteins, and in doing so define specificity of phosphotyrosine (pTyr) signalling networks. SH2 domains and protein-tyrosine phosphatases expand alongside protein-tyrosine kinases (PTKs) to coordinate cellular and organismal complexity in the evolution of the unikont branch of the eukaryotes. Examination of conserved families of PTKs and SH2 domain proteins provides fiduciary marks that trace the evolutionary landscape for the development of complex cellular systems in the proto-metazoan and metazoan lineages. The evolutionary provenance of conserved SH2 and PTK families reveals the mechanisms by which diversity is achieved through adaptations in tissue-specific gene transcription, altered ligand binding, insertions of linear motifs and the gain or loss of domains following gene duplication. We discuss mechanisms by which pTyr-mediated signalling networks evolve through the development of novel and expanded families of SH2 domain proteins and the elaboration of connections between pTyr-signalling proteins. These changes underlie the variety of general and specific signalling networks that give rise to tissue-specific functions and increasingly complex developmental programmes. Examination of SH2 domains from an evolutionary perspective provides insight into the process by which evolutionary expansion and modification of molecular protein interaction domain proteins permits the development of novel protein-interaction networks and accommodates adaptation of signalling networks.

Modular evolution of phosphorylation-based signalling systems

Phosphorylation sites are formed by protein kinases ('writers'), frequently exert their effects following recognition by phospho-binding proteins ('readers') and are removed by protein phosphatases ('erasers'). This writer-reader-eraser toolkit allows phosphorylation events to control a broad range of regulatory processes, and has been pivotal in the evolution of new functions required for the development of multi-cellular animals. The proteins that comprise this system of protein kinases, phospho-binding targets and phosphatases are typically modular in organization, in the sense that they are composed of multiple globular domains and smaller peptide motifs with binding or catalytic properties. The linkage of these binding and catalytic modules in new ways through genetic recombination, and the selection of particular domain combinations, has promoted the evolution of novel, biologically useful processes. Conversely, the joining of domains in aberrant combinations can subvert cell signalling and be causative in diseases such as cancer. Major inventions such as phosphotyrosine (pTyr)-mediated signalling that flourished in the first multi-cellular animals and their immediate predecessors resulted from stepwise evolutionary progression. This involved changes in the binding properties of interaction domains such as SH2 and their linkage to new domain types, and alterations in the catalytic specificities of kinases and phosphatases. This review will focus on the modular aspects of signalling networks and the mechanism by which they may have evolved.

This Déjà vu feeling--analysis of multidomain protein evolution in eukaryotic genomes

Evolutionary innovation in eukaryotes and especially animals is at least partially driven by genome rearrangements and the resulting emergence of proteins with new domain combinations, and thus potentially novel functionality. Given the random nature of such rearrangements, one could expect that proteins with particularly useful multidomain combinations may have been rediscovered multiple times by parallel evolution. However, existing reports suggest a minimal role of this phenomenon in the overall evolution of eukaryotic proteomes. We assembled a collection of 172 complete eukaryotic genomes that is not only the largest, but also the most phylogenetically complete set of genomes analyzed so far. By employing a maximum parsimony approach to compare repertoires of Pfam domains and their combinations, we show that independent evolution of domain combinations is significantly more prevalent than previously thought. Our results indicate that about 25% of all currently observed domain combinations have evolved multiple times. Interestingly, this percentage is even higher for sets of domain combinations in individual species, with, for instance, 70% of the domain combinations found in the human genome having evolved independently at least once in other species. We also show that previous, much lower estimates of this rate are most likely due to the small number and biased phylogenetic distribution of the genomes analyzed. The process of independent emergence of identical domain combination is widespread, not limited to domains with specific functional categories. Besides data from large-scale analyses, we also present individual examples of independent domain combination evolution. The surprisingly large contribution of parallel evolution to the development of the domain combination repertoire in extant genomes has profound consequences for our understanding of the evolution of pathways and cellular processes in eukaryotes and for comparative functional genomics.

Most proteins in eukaryotes are composed of two or more domains, evolutionary independent units with (often) their own individual functions. The specific repertoire of multidomain proteins in a given species defines the topology of pathways and networks that carry out its metabolic and regulatory processes. When proteins with new domain combinations emerge by gene fusion and fission, it directly affects topology of cellular networks in this organism. To better understand the evolution of such networks we analyzed a large set of eukaryotic genomes for the evolutionary history of known domain combinations. Our analysis shows that 70% of all domain combinations present in the human genome independently appeared in at least one other eukaryotic genome. Overall, over 25% of all known multidomain architectures emerged independently several times in the history of life. The difference between a global and species specific picture can be explained by the existence of a core set of domain combinations that keeps reemerging in different species, which are accompanied by a smaller number of unique domain combinations that do not appear anywhere else.

Phosphotyrosine recognition domains: the typical, the atypical and the versatile

SH2 domains are long known prominent players in the field of phosphotyrosine recognition within signaling protein networks. However, over the years they have been joined by an increasing number of other protein domain families that can, at least with some of their members, also recognise pTyr residues in a sequence-specific context. This superfamily of pTyr recognition modules, which includes substantial fractions of the PTB domains, as well as much smaller, or even single member fractions like the HYB domain, the PKCδ and PKCθ C2 domains and RKIP, represents a fascinating, medically relevant and hence intensely studied part of the cellular signaling architecture of metazoans. Protein tyrosine phosphorylation clearly serves a plethora of functions and pTyr recognition domains are used in a similarly wide range of interaction modes, which encompass, for example, partner protein switching, tandem recognition functionalities and the interaction with catalytically active protein domains. If looked upon closely enough, virtually no pTyr recognition and regulation event is an exact mirror image of another one in the same cell. Thus, the more we learn about the biology and ultrastructural details of pTyr recognition domains, the more does it become apparent that nature cleverly combines and varies a few basic principles to generate a sheer endless number of sophisticated and highly effective recognition/regulation events that are, under normal conditions, elegantly orchestrated in time and space. This knowledge is also valuable when exploring pTyr reader domains as diagnostic tools, drug targets or therapeutic reagents to combat human diseases.

High-throughput analysis of peptide-binding modules

Modular protein interaction domains (PIDs) that recognize linear peptide motifs are found in hundreds of proteins within the human genome. Some PIDs such as SH2, 14-3-3, Chromo, and Bromo domains serve to recognize posttranslational modification (PTM) of amino acids (such as phosphorylation, acetylation, methylation, etc.) and translate these into discrete cellular responses. Other modules such as SH3 and PSD-95/Discs-large/ZO-1 (PDZ) domains recognize linear peptide epitopes and serve to organize protein complexes based on localization and regions of elevated concentration. In both cases, the ability to nucleate-specific signaling complexes is in large part dependent on the selectivity of a given protein module for its cognate peptide ligand. High-throughput (HTP) analysis of peptide-binding domains by peptide or protein arrays, phage display, mass spectrometry, or other HTP techniques provides new insight into the potential protein-protein interactions prescribed by individual or even whole families of modules. Systems level analyses have also promoted a deeper understanding of the underlying principles that govern selective protein-protein interactions and how selectivity evolves. Lastly, there is a growing appreciation for the limitations and potential pitfalls associated with HTP analysis of protein-peptide interactomes. This review will examine some of the common approaches utilized for large-scale studies of PIDs and suggest a set of standards for the analysis and validation of datasets from large-scale studies of peptide-binding modules. We will also highlight how data from large-scale studies of modular interaction domain families can provide insight into systems level properties such as the linguistics of selective interactions.

Complex evolutionary history and diverse domain organization of SET proteins suggest divergent regulatory interactions

• Plants and animals possess very different developmental processes, yet share conserved epigenetic regulatory mechanisms, such as histone modifications. One of the most important forms of histone modification is methylation on lysine residues of the tails, carried out by members of the SET protein family, which are widespread in eukaryotes. • We analyzed molecular evolution by comparative genomics and phylogenetics of the SET genes from plant and animal genomes, grouping SET genes into several subfamilies and uncovering numerous gene duplications, particularly in the Suv, Ash, Trx and E(z) subfamilies. • Domain organizations differ between different subfamilies and between plant and animal SET proteins in some subfamilies, and support the grouping of SET genes into seven main subfamilies, suggesting that SET proteins have acquired distinctive regulatory interactions during evolution. We detected evidence for independent evolution of domain organization in different lineages, including recruitment of new domains following some duplications. • More recent duplications in both vertebrates and land plants are probably the result of whole-genome or segmental duplications. The evolution of the SET gene family shows that gene duplications caused by segmental duplications and other mechanisms have probably contributed to the complexity of epigenetic regulation, providing insights into the evolution of the regulation of chromatin structure.

The human phosphatase interactome: An intricate family portrait

The concerted activities of kinases and phosphatases modulate the phosphorylation levels of proteins, lipids and carbohydrates in eukaryotic cells. Despite considerable effort, we are still missing a holistic picture representing, at a proteome level, the functional relationships between kinases, phosphatases and their substrates. Here we focus on phosphatases and we review and integrate the available information that helps to place the members of the protein phosphatase superfamilies into the human protein interaction network. In addition we show how protein interaction domains and motifs, either covalently linked to the phosphatase domain or in regulatory/adaptor subunits, play a prominent role in substrate selection.

NHERF1 acts as a molecular switch to program metastatic behavior and organotropism via its PDZ domains

Tumor metastasis is the primary cause of death in cancer patients, but the molecular mechanisms driving the evolution of the phenotype toward a specific organ is one of its less understood aspects. The scaffolding protein NHERF1 reprograms the metastatic phenotype and organotropism via the differential function of its PDZ domains.

Metastatic cells are highly plastic for differential expression of tumor phenotype hallmarks and metastatic organotropism. The signaling proteins orchestrating the shift of one cell phenotype and organ pattern to another are little known. Na⁺/H⁺ exchanger regulatory factor (NHERF1) is a molecular pathway organizer, PDZ-domain protein that recruits membrane, cytoplasmic, and cytoskeletal signaling proteins into functional complexes. To gain insight into the role of NHERF1 in metastatic progression, we stably transfected a metastatic breast cell line, MDA-MB-231, with an empty vector, with wild-type NHERF1, or with NHERF1 mutated in either the PDZ1- or PDZ2-binding domains to block their binding activities. We observed that NHERF1 differentially regulates the expression of two phenotypic programs through its PDZ domains, and these programs form the mechanistic basis for metastatic organotropism. The PDZ2 domain promotes visceral metastases via increased invadopodia-dependent invasion and anchorage-independent growth, as well as by inhibition of apoptosis, whereas the PDZ1 domain promotes bone metastases by stimulating podosome nucleation, motility, neoangiogenesis, vasculogenic mimicry, and osteoclastogenesis in the absence of increased growth or invasion. Collectively, these findings identify NHERF1 as an important signaling nexus for coordinating cell structure with metastatic behavior and identifies the “mesenchymal-to-vasculogenic” phenotypic transition as an essential step in metastatic progression.

Subcellular organization: change of phase in partitioning the cellular milieu

Spatial organization and segregation are essential for the function of a complex and crowded cellular machine. New work demonstrates liquid-gel phase separation, both in vitro and in vivo, driven by the valency of constituent proteins.

Phase transitions in the assembly of multivalent signalling proteins

Cells are organized on length scales ranging from Angstroms to microns. However, the mechanisms by which Angstrom-scale molecular properties are translated to micron-scale macroscopic properties are not well understood. Here we show that interactions between diverse, synthetic multivalent macromolecules (including multi-domain proteins and RNA) produce sharp, liquid-liquid demixing phase separations, generating micron-sized liquid droplets in aqueous solution. This macroscopic transition corresponds to a molecular transition between small complexes and large, dynamic supramolecular polymers. The concentrations needed for phase transition are directly related to valency of the interacting species. In the case of the actin regulatory protein, neuronal Wiskott-Aldrich Syndrome Protein (N-WASP) interacting with its established biological partners Nck and phosphorylated nephrin¹, the phase transition corresponds to a sharp increase in activity toward the actin nucleation factor, Arp2/3 complex. The transition is governed by the degree of phosphorylation of nephrin, explaining how this property of the system can be controlled to regulatory effect by kinases. The widespread occurrence of multivalent systems suggests that phase transitions are likely used to spatially organize and biochemically regulate information throughout biology.

Rewiring of PDZ domain-ligand interaction network contributed to eukaryotic evolution

PDZ domain-mediated interactions have greatly expanded during metazoan evolution, becoming important for controlling signal flow via the assembly of multiple signaling components. The evolutionary history of PDZ domain-mediated interactions has never been explored at the molecular level. It is of great interest to understand how PDZ domain-ligand interactions emerged and how they become rewired during evolution. Here, we constructed the first human PDZ domain-ligand interaction network (PDZNet) together with binding motif sequences and interaction strengths of ligands. PDZNet includes 1,213 interactions between 97 human PDZ proteins and 591 ligands that connect most PDZ protein-mediated interactions (98%) in a large single network via shared ligands. We examined the rewiring of PDZ domain-ligand interactions throughout eukaryotic evolution by tracing changes in the C-terminal binding motif sequences of the PDZ ligands. We found that interaction rewiring by sequence mutation frequently occurred throughout evolution, largely contributing to the growth of PDZNet. The rewiring of PDZ domain-ligand interactions provided an effective means of functional innovations in nervous system development. Our findings provide empirical evidence for a network evolution model that highlights the rewiring of interactions as a mechanism for the development of new protein functions. PDZNet will be a valuable resource to further characterize the organization of the PDZ domain-mediated signaling proteome.

Rewiring of interactions is a powerful tool for the evolution of organism complexity. Rewiring among preexisting proteins provides a simple mechanism for the development of new signaling circuits by redirecting information flows without a gain or loss of genes. Particularly, interactions mediated by short linear motifs can be easily changed by mutations during evolution, resulting in a rewiring of interactions. However, how interaction rewiring of linear motif interactions facilitates the emergence of new protein function during evolution is poorly understood. Here, we systematically investigated the rewiring of interactions mediated by PDZ domains, which are one of the most commonly found peptide recognition modules. We found that PDZ domain-ligand interactions are frequently rewired by C-terminal sequence mutations in PDZ ligands during evolution. Especially, rewiring of PDZ domain-ligand interactions was involved in neuronal function development, occurring concurrently with the emergence of vertebrates and suggesting that reorganization of signaling pathways by rewiring PDZ domain-ligand interactions significantly contributed to the evolution of nervous systems in vertebrates. Our findings highlight the rewiring of interactions as an effective means for functional innovation, providing new insight into eukaryotic evolution, which has not been fully explained by only the expansion of protein families.

Conserved BK channel-protein interactions reveal signals relevant to cell death and survival

The large-conductance Ca²⁺-activated K⁺ (BK) channel and its β-subunit underlie tuning in non-mammalian sensory or hair cells, whereas in mammals its function is less clear. To gain insights into species differences and to reveal putative BK functions, we undertook a systems analysis of BK and BK-Associated Proteins (BKAPS) in the chicken cochlea and compared these results to other species. We identified 110 putative partners from cytoplasmic and membrane/cytoskeletal fractions, using a combination of coimmunoprecipitation, 2-D gel, and LC-MS/MS. Partners included 14-3-3γ, valosin-containing protein (VCP), stathmin (STMN), cortactin (CTTN), and prohibitin (PHB), of which 16 partners were verified by reciprocal coimmunoprecipitation. Bioinformatics revealed binary partners, the resultant interactome, subcellular localization, and cellular processes. The interactome contained 193 proteins involved in 190 binary interactions in subcellular compartments such as the ER, mitochondria, and nucleus. Comparisons with mice showed shared hub proteins that included N-methyl-D-aspartate receptor (NMDAR) and ATP-synthase. Ortholog analyses across six species revealed conserved interactions involving apoptosis, Ca²⁺ binding, and trafficking, in chicks, mice, and humans. Functional studies using recombinant BK and RNAi in a heterologous expression system revealed that proteins important to cell death/survival, such as annexinA5, γ-actin, lamin, superoxide dismutase, and VCP, caused a decrease in BK expression. This revelation led to an examination of specific kinases and their effectors relevant to cell viability. Sequence analyses of the BK C-terminus across 10 species showed putative binding sites for 14-3-3, RAC-α serine/threonine-protein kinase 1 (Akt), glycogen synthase kinase-3β (GSK3β) and phosphoinositide-dependent kinase-1 (PDK1). Knockdown of 14-3-3 and Akt caused an increase in BK expression, whereas silencing of GSK3β and PDK1 had the opposite effect. This comparative systems approach suggests conservation in BK function across different species in addition to novel functions that may include the initiation of signals relevant to cell death/survival.

The SH2 domain-containing proteins in 21 species establish the provenance and scope of phosphotyrosine signaling in eukaryotes

The Src homology 2 (SH2) domains are participants in metazoan signal transduction, acting as primary mediators for regulated protein-protein interactions with tyrosine-phosphorylated substrates. Here, we describe the origin and evolution of SH2 domain proteins by means of sequence analysis from 21 eukaryotic organisms from the basal unicellular eukaryotes, where SH2 domains first appeared, through the multicellular animals and increasingly complex metazoans. On the basis of our results, SH2 domains and phosphotyrosine signaling emerged in the early Unikonta, and the numbers of SH2 domains expanded in the choanoflagellate and metazoan lineages with the development of tyrosine kinases, leading to rapid elaboration of phosphotyrosine signaling in early multicellular animals. Our results also indicated that SH2 domains coevolved and the number of the domains expanded alongside protein tyrosine kinases and tyrosine phosphatases, thereby coupling phosphotyrosine signaling to downstream signaling networks. Gene duplication combined with domain gain or loss produced novel SH2-containing proteins that function within phosphotyrosine signaling, which likely have contributed to diversity and complexity in metazoans. We found that intra- and intermolecular interactions within and between SH2 domain proteins increased in prevalence along with organismal complexity and may function to generate more highly connected and robust phosphotyrosine signaling networks.

An antibody to de-N-acetyl sialic acid containing-polysialic acid identifies an intracellular antigen and induces apoptosis in human cancer cell lines

Polysialic acid (PSA), an α2,8-linked homopolymer of N-acetylneuraminic acid (Neu5Ac), is developmentally regulated and its expression is thought to be restricted to a few tissues in adults. Recently, we showed that two human pathogens expressed a derivative of PSA containing de-N-acetyl sialic acid residues (NeuPSA). Here we show that an epitope identified by the anti-NeuPSA monoclonal antibody, SEAM 3 (SEAM 3-reactive antigen or S3RA), is expressed in human melanomas, and also intracellularly in a human melanoma cell line (SK-MEL-28), a human T cell leukemia cell line (Jurkat), and two neuroblastoma cell lines (CHP-134 and SH-SY5Y). SEAM 3 binding induced apoptosis in the four cell lines tested. The unusual intracellular distribution of S3RA was similar to that described for the PSA polysialyltransferases, STX and PST, which are also expressed in the four cell lines used here. Interestingly, suppression of PST mRNA expression by transfection of SK-MEL-28 cells with PST-specific short interfering RNA (siRNA) resulted in decreased SEAM 3 binding. The results suggest further studies of the utility of antibodies such as SEAM 3 as therapeutic agents for certain malignancies.

Evolution and quantitative comparison of genome-wide protein domain distributions

The metabolic and regulatory capabilities of an organism are implicit in its protein content. This is often hard to estimate, however, due to ascertainment biases inherent in the available genome annotations. Its complement of recognizable functional protein domains and their combinations convey essentially the same information and at the same time are much more readily accessible, although protein domain models trained for one phylogenetic group frequently fail on distantly related sequences. Pooling related domain models based on their GO-annotation in combination with de novo gene prediction methods provides estimates that seem to be less affected by phylogenetic biases. We show here for 18 diverse representatives from all eukaryotic kingdoms that a pooled analysis of the tendencies for co-occurrence or avoidance of protein domains is indeed feasible. This type of analysis can reveal general large-scale patterns in the domain co-occurrence and helps to identify lineage-specific variations in the evolution of protein domains. Somewhat surprisingly, we do not find strong ubiquitous patterns governing the evolutionary behavior of specific functional classes. Instead, there are strong variations between the major groups of Eukaryotes, pointing at systematic differences in their evolutionary constraints.

Deciphering arginine methylation: Tudor tells the tale

Proteins can be modified by post-translational modifications such as phosphorylation, methylation, acetylation and ubiquitylation, creating binding sites for specific protein domains. Methylation has pivotal roles in the formation of complexes that are involved in cellular regulation, including in the generation of small RNAs. Arginine methylation was discovered half a century ago, but the ability of methylarginine sites to serve as binding motifs for members of the Tudor protein family, and the functional significance of the protein-protein interactions that are mediated by Tudor domains, has only recently been appreciated. Tudor proteins are now known to be present in PIWI complexes, where they are thought to interact with methylated PIWI proteins and regulate the PIWI-interacting RNA (piRNA) pathway in the germ line.

Evolutionary versatility of eukaryotic protein domains revealed by their bigram networks

Background

Protein domains are globular structures of independently folded polypeptides that exert catalytic or binding activities. Their sequences are recognized as evolutionary units that, through genome recombination, constitute protein repertoires of linkage patterns. Via mutations, domains acquire modified functions that contribute to the fitness of cells and organisms. Recent studies have addressed the evolutionary selection that may have shaped the functions of individual domains and the emergence of particular domain combinations, which led to new cellular functions in multi-cellular animals. This study focuses on modeling domain linkage globally and investigates evolutionary implications that may be revealed by novel computational analysis.

Results

A survey of 77 completely sequenced eukaryotic genomes implies a potential hierarchical and modular organization of biological functions in most living organisms. Domains in a genome or multiple genomes are modeled as a network of hetero-duplex covalent linkages, termed bigrams. A novel computational technique is introduced to decompose such networks, whereby the notion of domain "networking versatility" is derived and measured. The most and least "versatile" domains (termed "core domains" and "peripheral domains" respectively) are examined both computationally via sequence conservation measures and experimentally using selected domains. Our study suggests that such a versatility measure extracted from the bigram networks correlates with the adaptivity of domains during evolution, where the network core domains are highly adaptive, significantly contrasting the network peripheral domains.

Conclusions

Domain recombination has played a major part in the evolution of eukaryotes attributing to genome complexity. From a system point of view, as the results of selection and constant refinement, networks of domain linkage are structured in a hierarchical modular fashion. Domains with high degree of networking versatility appear to be evolutionary adaptive, potentially through functional innovations. Domain bigram networks are informative as a model of biological functions. The networking versatility indices extracted from such networks for individual domains reflect the strength of evolutionary selection that the domains have experienced.

The plexus model for the inference of ancestral multidomain proteins

Interactions of protein domains control essential cellular processes. Thus, inferring the evolutionary histories of multidomain proteins in the context of their families can provide rewarding insights into protein function. However, methods to infer these histories are challenged by the complexity of macroevolutionary events. Here, we address this challenge by describing an algorithm that computes a novel network-like structure, called plexus, which represents the evolution of domains and their combinations. Finally, we demonstrate the performance of this algorithm with empirical data sets.

Processes of fungal proteome evolution and gain of function: gene duplication and domain rearrangement

During evolution, organisms have gained functional complexity mainly by modifying and improving existing functioning systems rather than creating new ones ab initio. Here we explore the interplay between two processes which during evolution have had major roles in the acquisition of new functions: gene duplication and protein domain rearrangements. We consider four possible evolutionary scenarios: gene families that have undergone none of these event types; only gene duplication; only domain rearrangement, or both events. We characterize each of the four evolutionary scenarios by functional attributes. Our analysis of ten fungal genomes indicates that at least for the fungi clade, species significantly appear to gain complexity by gene duplication accompanied by the expansion of existing domain architectures via rearrangements. We show that paralogs gaining new domain architectures via duplication tend to adopt new functions compared to paralogs that preserve their domain architectures. We conclude that evolution of protein families through gene duplication and domain rearrangement is correlated with their functional properties. We suggest that in general, new functions are acquired via the integration of gene duplication and domain rearrangements rather than each process acting independently.

Evolving specificity from variability for protein interaction domains

An important question in modular domain-peptide interactions, which play crucial roles in many biological processes, is how the diverse specificities exhibited by different members of a domain family are encoded in a common scaffold. Analysis of the Src homology (SH) 2 family has revealed that its specificity is determined, in large part, by the configuration of surface loops that regulate ligand access to binding pockets. In a distinct manner, SH3 domains employ loops for ligand recognition. The PDZ domain, in contrast, achieves specificity by co-evolution of binding-site residues. Thus, the conformational and sequence variability afforded by surface loops and binding sites provides a general mechanism by which to encode the wide spectrum of specificities observed for modular protein interaction domains.

Keeping it in the family: diverse histone recognition by conserved structural folds

Epigenetic regulation of gene transcription relies on an array of recurring structural domains that have evolved to recognize post-translational modifications on histones. The roles of bromodomains, PHD fingers, and the Royal family domains in the recognition of histone modifications to direct transcription have been well characterized. However, only through recent structural studies has it been realized that these basic folds are capable of interacting with increasingly more complex histone modification landscapes, illuminating how nature has concocted a way to accomplish more with less. Here we review the recent biochemical and structural studies of several conserved folds that recognize modified as well as unmodified histone sequences, and discuss their implications on gene expression.

Tandem BRCT Domains: DNA's Praetorian Guard

The cell's ability to sense and respond to specific stimuli is a complex system derived from precisely regulated protein-protein interactions. Some of these protein-protein interactions are mediated by the recognition of linear peptide motifs by protein modular domains. BRCT (BRCA1 C-terminal) domains and their linear motif counterparts, which contain phosphoserines, are one such pair-wise interaction system that seems to have evolved to serve as a surveillance system to monitor threats to the cell's genetic integrity. Evidence indicates that BRCT domains found in tandem can cooperate to provide sequence-specific binding of phosphorylated peptides as is the case for the breast and ovarian cancer susceptibility gene BRCA1 and the PAX transcription factor-interacting protein PAXIP1. Particular interest has been paid to tandem BRCT domains as "readers" of signaling events in the form of phosphorylated serine moieties induced by the activation of DNA damage response kinases ATM, ATR, and DNA-PK. However, given the diversity of tandem BRCT-containing proteins, questions remain as to the origin and evolution of this domain. Here, we discuss emerging views of the origin and evolving roles of tandem BRCT domain repeats in the DNA damage response.

Structural basis for recognition of arginine methylated Piwi proteins by the extended Tudor domain

Arginine methylation modulates diverse cellular processes and represents a molecular signature of germ-line-specific Piwi family proteins. A subset of Tudor domains recognize arginine methylation modifications, but the binding mechanism has been lacking. Here we establish that, like other germ-line Tudor proteins, the ancestral staphylococcal nuclease domain-containing 1 (SND1) polypeptide is expressed and associates with PIWIL1/Miwi in germ cells. We find that human SND1 binds PIWIL1 in an arginine methylation-dependent manner with a preference for symmetrically dimethylated arginine. The entire Tudor domain and a bifurcated SN domain are required for this binding activity, whereas the canonical Tudor domain alone is insufficient for methylarginine ligand binding. Crystal structures show that the intact SND1 extended Tudor domain forms a wide and negatively charged binding groove, which can accommodate distinct symmetrically dimethylated arginine peptides from PIWIL1 in different orientations. This analysis explains how SND1 preferentially recognizes symmetrical dimethylarginine via an aromatic cage and conserved hydrogen bonds, and provides a general paradigm for the binding mechanisms of methylarginine-containing peptides by extended Tudor domains.

Phosphotyrosine signaling: evolving a new cellular communication system

Tyrosine phosphorylation controls many cellular functions. Yet the three-part toolkit that regulates phosphotyrosine signaling-tyrosine kinases, phosphotyrosine phosphatases, and Src Homology 2 (SH2) domains-is a relatively new innovation. Genomic analyses reveal how this revolutionary signaling system may have originated and why it rapidly became critical to metazoans.

Identity and divergence of protein domain architectures after the yeast whole-genome duplication event

Gene duplication is a key mechanism in evolution for generating new functionality, and it is known to have produced a large proportion of genes. Duplication mechanisms include small-scale, or "local", events such as unequal crossing over and retroposition, together with global events, such as chromosomal or whole genome duplication (WGD). In particular, different studies confirmed that the yeast S. cerevisiae arose from a 100-150 million-year old whole-genome duplication. Detection and study of duplications are usually based on sequence alignment, synteny and phylogenetic techniques, but protein domains are also useful in assessing protein homology. We develop a simple and computationally efficient protein domain architecture comparison method based on the domain assignments available from public databases. We test the accuracy and the reliability of this method in detecting instances of gene duplication in the yeast S. cerevisiae. In particular, we analyze the evolution of WGD and non-WGD paralogs from the domain viewpoint, in comparison with a more standard functional analysis of the genes. A large number of domains is shared by genes that underwent local and global duplications, indicating the existence of a common set of "duplicable" domains. On the other hand, WGD and non-WGD paralogs tend to have different functions. We find evidence that this comes from functional migration within similar domain superfamilies, but also from the existence of small sets of WGD and non-WGD specific domain superfamilies with largely different functions. This observation gives a novel perspective on the finding that WGD paralogs tend to be functionally different from small-scale paralogs. WGD and non-WGD superfamilies carry distinct functions. Finally, the Gene Ontology similarity of paralogs tends to decrease with duplication age, while this tendency is weaker or not observable by the comparison of the domain architectures of paralogs. This suggests that the set of domains composing a protein tends to be maintained, while its function, cellular process or localization diversifies. Overall, the gathered evidence gives a different viewpoint on the biological specificity of the WGD and at the same time points out the validity of domain architecture comparison as a tool for detecting homology.

Building RNA-protein granules: insight from the germline

The germline originates from primordial embryonic germ cells which give rise to sperm and egg cells and consequently, to the next generation. Germ cells of many organisms contain electron-dense granules that comprise RNA and proteins indispensable for germline development. Here we review recent reports that provide important insights into the structure and function of crucial RNA and protein components of the granules, including DEAD-box helicases, Tudor domain proteins, Piwi/Argonaute proteins and piRNA. Collectively, these components function in translational control, remodeling of ribonucleoprotein complexes and transposon silencing. Furthermore, they interact with each other by means of conserved structural modules and post-translationally modified amino acids. These data suggest a widespread use of several protein motifs in germline development and further our understanding of other ribonucleoprotein structures, for example, processing bodies and neuronal granules.

Science Signaling Podcast: 24 November 2009

Bioinformatics analysis reveals how protein domain composition correlates with evolutionary change.