Exons as microgenes?

Tracing primordial protein evolution through structurally guided stepwise segment elongation

The understanding of how primordial proteins emerged has been a fundamental and longstanding issue in biology and biochemistry. For a better understanding of primordial protein evolution, we synthesized an artificial protein on the basis of an evolutionary hypothesis, segment-based elongation starting from an autonomously foldable short peptide. A 10-residue protein, chignolin, the smallest foldable polypeptide ever reported, was used as a structural support to facilitate higher structural organization and gain-of-function in the development of an artificial protein. Repetitive cycles of segment elongation and subsequent phage display selection successfully produced a 25-residue protein, termed AF.2A1, with nanomolar affinity against the Fc region of immunoglobulin G. AF.2A1 shows exquisite molecular recognition ability such that it can distinguish conformational differences of the same molecule. The structure determined by NMR measurements demonstrated that AF.2A1 forms a globular protein-like conformation with the chignolin-derived β-hairpin and a tryptophan-mediated hydrophobic core. Using sequence analysis and a mutation study, we discovered that the structural organization and gain-of-function emerged from the vicinity of the chignolin segment, revealing that the structural support served as the core in both structural and functional development. Here, we propose an evolutionary model for primordial proteins in which a foldable segment serves as the evolving core to facilitate structural and functional evolution. This study provides insights into primordial protein evolution and also presents a novel methodology for designing small sized proteins useful for industrial and pharmaceutical applications.

Structural diversity of protein segments follows a power-law distribution

The local structures of protein segments were classified and their distribution was analyzed to explore the structural diversity of proteins. Representative proteins were divided into short segments using a sliding L-residue window. Each set of local structures consisting of consecutive 1-31 amino acids was classified using a single-pass clustering method. The results demonstrate that the local structures of proteins are very unevenly distributed in the protein universe. The distribution of local structures of relatively long segments shows a power-law behavior that is formulated well by Zipf's law, implying that a protein structure possesses recursive and fractal characteristics. The degree of effective conformational freedom per residue as well as the structure entropy per residue decreases gradually with an increasing value of L and then converges to constant values. This suggests that the number of protein conformations resides within the range between 1.2L and 1.5L and that 10- to 20-residue segments are already proteinlike in terms of their structural diversity.

10 Residue Folded Peptide Designed by Segment Statistics

We have designed a peptide termed chignolin, consisting of only 10 amino acid residues (GYDPETGTWG), on the basis of statistics derived from more than 10,000 protein segments. The peptide folds into a unique structure in water and shows a cooperative thermal transition, both of which may be hallmarks of a protein. Also, the experimentally determined beta-hairpin structure was very close to what we had targeted. The performance of the short peptide not only implies that the methodology employed here can contribute toward development of novel techniques for protein design, but it also yields insights into the raison d'etre of an autonomous element involved in a natural protein. This is of interest for the pursuit of folding mechanisms and evolutionary processes of proteins.

On the structural and functional modularity of glycinamide ribonucleotide formyltransferases

Glycinamide ribonucleotide formyltransferases (GARTs) are part of the de novo purine biosynthetic pathway, catalyzing the direct transfer of a formyl group from the tetrahydrofolate cofactor to the glycinamide ribonucleotide substrate. Despite the low amino acid-sequence identity between the GARTs from Escherichia coli and human, their tertiary structures are superimposable. As part of our functional studies of these enzymes, we have investigated the interchangeability of individual protein fragments or modules between the two enzymes and the functional properties of the resulting hybrids. The modular nature of GART facilitated the creation of combinatorial libraries of chimeras between the Escherichia coli and human enzymes, which were functionally selected through complementation of an auxotrophic Escherichia coli strain. From a pool of several dozen sequence distinct hybrids, six in vivo-functional fusion genes were selected, overexpressed, and purified to homogeneity. The kinetic analysis of these constructs and the comparison of their k(cat) and K(M) values to the parental enzymes suggest that the characteristic kinetic properties from the two parents are "modular encoded" and can be exchanged by domain swapping. The chimeras in general, however, are subject to temperature instability and misfolding; thus, they serve primarily as useful candidates for further rounds of optimization.

Scale-free behavior in protein domain networks

Several technical, social, and biological networks were recently found to demonstrate scale-free and small-world behavior instead of random graph characteristics. In this work, the topology of protein domain networks generated with data from the ProDom, Pfam, and Prosite domain databases was studied. It was found that these networks exhibited small-world and scale-free topologies with a high degree of local clustering accompanied by a few long-distance connections. Moreover, these observations apply not only to the complete databases, but also to the domain distributions in proteomes of different organisms. The extent of connectivity among domains reflects the evolutionary complexity of the organisms considered.

Testing the role of chain connectivity on the stability and structure of dihydrofolate reductase from E. coli: fragment complementation and circular permutation reveal stable, alternatively folded forms

The effects of chain cleavage and circular permutation on the structure, stability, and activity of dihydrofolate reductase (DHFR) from Escherichia coli were investigated by various spectroscopic and biochemical methods. Cleavage of the backbone after position 86 resulted in two fragments, (1--86) and (87--159) each of which are poorly structured and enzymatically inactive. When combined in a 1 : 1 molar ratio, however, the fragments formed a high-affinity (K(a) = 2.6 x 10(7) M(-1)) complex that displays a weakly cooperative urea-induced unfolding transition at micromolar concentrations. The retention of about 15% of the enzymatic activity of full-length DHFR is surprising, considering that the secondary structure in the complex is substantially reduced from its wild-type counterpart. In contrast, a circularly permuted form with its N-terminus at position 86 has similar overall stability to full-length DHFR, about 50% of its activity, substantial secondary structure, altered side-chain packing in the adenosine binding domain, and unfolds via an equilibrium intermediate not observed in the wild-type protein. After addition of ligand or the tight-binding inhibitor methotrexate, both the fragment complex and the circular permutant adopt more native-like secondary and tertiary structures. These results show that changes in the backbone connectivity can produce alternatively folded forms and highlight the importance of protein-ligand interactions in stabilizing the active site architecture of DHFR.

The evolutionary transition from monomeric to oligomeric proteins: tools, the environment, hypotheses

Recently, renewed interest in the evolution of oligomeric proteins has seen new approaches explored and new hypotheses proposed. The model systems chosen are generally made up of pairs of homologous proteins, each composed of a monomer and a dimeric counterpart, but the question has been also approached by comparing statistically significant structural patterns in sets of monomeric and oligomeric proteins. Here the tools of genetics and chemistry potentially available to the evolution of oligomeric proteins are discussed, as well as the possible effects of environments on the early attempts to oligomerization. Traces of an ancestral monomeric status of oligomers may be detected in the significant presence of polar and charged residues at intersubunit interfaces, and by the recognition that, besides the hydrophobic effect, a 'hydrophilic' effect has also had a role in the construction of these interfaces. The traditional 'mutation' model is described and found to be based on a hierarchy of mutations, crowned by a 'primary' mutation, one that could prime oligomerization by irreversibly altering the structure of an ancestral monomer. The mechanism of oligomerization based on the exchange or 'swap' of structural elements between monomers is discussed. The possibility is also discussed that the main steps in the folding pathway of an oligomeric protein reiterate the main steps in its evolution.

Towards a reconciliation of the introns early or late views: triosephosphate isomerase genes from insects

The gene encoding the glycolytic enzyme, triosephosphate isomerase (TPI; EC 5.3.1.1), is a favourite model for molecular evolutionists who either subscribe to the theory that introns co-evolved with the ancestral gene, the introns early view, or alternatively, that introns are more recent immigrants. The discovery of an intron in the TPI gene of Culex mosquitoes at a site which was predicted by proponents of the intron early school supported that theory. More recently, the discovery of additional intron sites in several eukaryotes was presented as evidence supporting the introns late school. We have found the 'Culex intron' in two closely related mosquitoes, but not in two more evolutionary primitive Dipterans, suggesting that, if it is an 'ancient intron', loss may be more frequent than that supposed by the intron late school. In addition, we have found that three introns punctuating the TPI gene from the Lepidopteran, Heliothis, appear to be ancestrally related and may be the result of transposable element insertion, 50-90 million years ago. It is argued that both opposing schools in the intron debate be reconciled -- some introns may have been early and certainly others have arrived subsequent to the appearance of the TPI gene.

A relationship between GC content and coding-sequence length

Since base composition of translational stop codons (TAG, TAA, and TGA) is biased toward a low G+C content, a differential density for these termination signals is expected in random DNA sequences of different base compositions. The expected length of reading frames (DNA segments of sense codons flanked by in-phase stop codons) in random sequences is thus a function of GC content. The analysis of DNA sequences from several genome databases stratified according to GC content reveals that the longest coding sequences-exons in vertebrates and genes in prokaryotes-are GC-rich, while the shortest ones are GC-poor. Exon lengthening in GC-rich vertebrate regions does not result, however, in longer vertebrate proteins, perhaps because of the lower number of exons in the genes located in these regions. The effects on coding-sequence lengths constitute a new evolutionary meaning for compositional variations in DNA GC content.

Foldons, protein structural modules, and exons

Foldons, which are kinetically competent, quasi-independently folding units of a protein, may be defined using energy landscape analysis. Foldons can be identified by maxima in a scan of the ratio of a contiguous segment's energetic stability gap to the energy variance of that segment's molten globule states, reflecting the requirement of minimal frustration. The predicted foldons are compared with the exons and structural modules for 16 of the 30 proteins studied. Statistical analysis indicates a strong correlation between the energetically determined foldons and Go's geometrically defined structural modules, but there are marked sequence-dependent effects. There is only a weak correlation of foldons to exons. For gammaII-crystallin, myoglobin, barnase, alpha-lactalbumin, and cytochrome c the foldons and some noncontiguous clusters of foldons compare well with intermediates observed in experiment.

Correlation of intron-exon organisation with the three-dimensional structure in glutamate dehydrogenase

The positions of the intron-exon boundaries in the genes for glutamate dehydrogenase from Chlorella sorokiniana rat, and human have been located on the three-dimensional structure of the highly homologous enzyme from Clostridium symbiosum and analysed for their position in the protein structure. This analysis shows no correlation between the positions of these boundaries in the mammalian and Chlorella glutamate dehydrogenase genes and no correlation with units of function in the enzyme and suggests that the present day exons do not represent the protein modules of an ancestral glutamate dehydrogenase. There appears to be no clear preference for the residues at the splice junctions to be either buried or exposed to solvent. However, the frequency with which the introns appear in the loops linking elements of secondary structure, rather than in either the alpha-helical or beta-sheet segments, is higher than predicted on the basis of the proportion of residues in the loops. This is consistent with but not proof of a role for exon modification/exchange in protein evolution since changes at these positions are less likely to disturb the structure and hence maintain function.

Structure of the human deoxyribonuclease I (DNase I) gene: identification of the nucleotide substitution that generates its classical genetic polymorphism

The objectives of this study were to elucidate the structural organization of the gene for human deoxyribonuclease I (DNase I) and to identify the mutation site underlying its classical genetic polymorphism. In order to determine the organization of this gene, we utilized a combination of direct polymerase chain reaction (PCR)-amplification of human genomic DNA and isolation of the overlapping clones from a cosmid human genomic library. Restriction endonuclease mapping, Southern blotting and DNA sequencing showed that the DNase I gene was approximately 3.2 kilobases long, it comprised 9 (I-IX) exons separated by eight introns and its complete sequence was determined. The first exon contained only the non-translated sequences of mRNA. In addition to several putative regulatory elements, TATA-like and CAAT-like sequences were observed in the region upstream of the translation initiation codon. These results provide information that will help to understand the expression and regulation of DNase I. The isoelectric focusing patterns of human DNase I showed that it exhibits classical genetic polymorphism (Kishi et al. 1989, 1990). A comparison of the entire translated sequences of the DNase I gene from two pairs of individuals with common DNase I phenotypes 1 and 2 revealed only one nucleotide residue difference in exon VIII, A for phenotype 1 and G for phenotype 2, thus producing Gln and Arg amino acid substitutions respectively at position 222 from the NH2-terminus of the mature enzyme. The predicted charge changes attributable to these amino acid substitutions are entirely consistent with the isoelectric focusing profiles of these two DNase I isozymes. We conclude that this substitution is solely responsible for the classical polymorphism of DNase I protein.

Steroid receptor domain conformations and hormone antagonism

Receptor stabilization, activation, dimerization, and binding to cognate sequences on DNA are possible with antagonists. Tissue-, steroid-, and species-dependent differences in all these parameters, despite identical structure of the receptor from various sources for any one steroid hormone class, suggest posttranslational modifications of a primary gene product. Clinically, it is now possible to visualize receptor-specific antihormone therapy of various steroid-dependent maladies (cancer of the breast, uterus, or prostate, Cushing's disease, hypertensive disorders, etc.) where surgical resection has been hitherto most effective. Amelioration of adverse side effects, associated with currently available semispecific derivatives, should permit wider applications in a variety of other situations in the near future.

Phosphoglucomutase 1: a gene with two promoters and a duplicated first exon

In view of its central role in glycolysis and gluconeogenesis and its polymorphic genetic variability, the phosphoglucomutase 1 (PGM1) gene in man has been the target of protein structural studies and genetic analysis for more than 25 years. We have now isolated genomic clones containing the complete PGM1 gene and have shown that it spans over 65 kb and contains 11 exons. We have also shown that the sites of the two mutations which form the molecular basis for the common PGM1 protein polymorphism lie in exons 4 and 8 and are 18 kb apart. Within this region there is a site of intragenic recombination. We have discovered two alternatively spliced first exons, one of which, exon 1A, is transcribed in a wide variety of cell types; the other, exon 1B, is transcribed in fast muscle. Exon 1A is transcribed from a promoter which has the structural hallmarks of a housekeeping promoter but lies more than 35 kb upstream of exon 2. Exon 1B lies 6 kb upstream of exon 2 within the large first intron of the ubiquitously expressed PGM1 transcript. The fast-muscle form of PGM1 is characterized by 18 extra amino acid residues at its N-terminal end. Sequence comparisons show that exons 1A and 1B are structurally related and have arisen by duplication.

Coding coenzyme handles: a hypothesis for the origin of the genetic code

The coding coenzyme handle hypothesis suggests that useful coding preceded translation. Early adapters, the ancestors of present-day anticodons, were charged with amino acids acting as coenzymes of ribozymes in a metabolically complex RNA world. The ancestral aminoacyl-adapter synthetases could have been similar to present-day self-splicing tRNA introns. A codon-anticodon-discriminator base complex embedded in these synthetases could have played an important role in amino acid recognition. Extension of the genetic code proceeded through the take-over of nonsense codons by novel amino acids, related to already coded ones either through precursor-product relationship or physicochemical similarity. The hypothesis is open for experimental tests.

Clinical and genetic patterns of neurofibromatosis 1 and 2

Images