Computational analysis of amino acids and their sidechain analogs in crowded solutions of RNA nucleobases with implications for the mRNA-protein complementarity hypothesis

Coding From Binding? Molecular Interactions at the Heart of Translation

The mechanism and the evolution of DNA replication and transcription, the key elements of the central dogma of biology, are fundamentally well explained by the physicochemical complementarity between strands of nucleic acids. However, the determinants that have shaped the third part of the dogma-the process of biological translation and the universal genetic code-remain unclear. We review and seek parallels between different proposals that view the evolution of translation through the prism of weak, noncovalent interactions between biological macromolecules. In particular, we focus on a recent proposal that there exists a hitherto unrecognized complementarity at the heart of biology, that between messenger RNA coding regions and the proteins that they encode, especially if the two are unstructured. Reflecting the idea that the genetic code evolved from intrinsic binding propensities between nucleotides and amino acids, this proposal promises to forge a link between the distant past and the present of biological systems.

Compositional complementarity between genomic RNA and coat proteins in positive-sense single-stranded RNA viruses

During packaging in positive-sense single-stranded RNA (+ssRNA) viruses, coat proteins (CPs) interact directly with multiple regions in genomic RNA (gRNA), but the underlying physicochemical principles remain unclear. Here we analyze the high-resolution cryo-EM structure of bacteriophage MS2 and show that the gRNA/CP binding sites, including the known packaging signal, overlap significantly with regions where gRNA nucleobase-density profiles match the corresponding CP nucleobase-affinity profiles. Moreover, we show that the MS2 packaging signal corresponds to the global minimum in gRNA/CP interaction energy in the unstructured state as derived using a linearly additive model and knowledge-based nucleobase/amino-acid affinities. Motivated by this, we predict gRNA/CP interaction sites for a comprehensive set of 1082 +ssRNA viruses. We validate our predictions by comparing them with site-resolved information on gRNA/CP interactions derived in SELEX and CLIP experiments for 10 different viruses. Finally, we show that in experimentally studied systems CPs frequently interact with autologous coding regions in gRNA, in agreement with both predicted interaction energies and a recent proposal that proteins in general tend to interact with own mRNAs, if unstructured. Our results define a self-consistent framework for understanding packaging in +ssRNA viruses and implicate interactions between unstructured gRNA and CPs in the process.

The Origin of Genetic Code and Translation in the Framework of Current Concepts on the Origin of Life

The origin of genetic code and translation system is probably the central and most difficult problem in the investigations on the origin of life and one of the most complex problems in the evolutionary biology in general. There are multiple hypotheses on the emergence and development of existing genetic systems that propose the mechanisms for the origin and early evolution of genetic code, as well as for the emergence of replication and translation. Here, we discuss the most well-known of these hypotheses, although none of them provides a description of the early evolution of genetic systems without gaps and assumptions. The RNA world hypothesis is a currently prevailing scientific idea on the early evolution of biological and pre-biological structures, the main advantage of which is the assumption that RNAs as the first living systems were self-sufficient, i.e., capable of functioning as both catalysts and templates. However, this hypothesis has also significant limitations. In particular, no ribozymes with processive polymerase activity have been yet discovered or synthesized. Taking into account the mutual need of proteins and nucleic acids in each other in the current world, many authors propose the early evolution scenarios based on the co-evolution of these two classes of organic molecules. They postulate that the emergence of translation was necessary for the replication of nucleic acids, in contrast to the RNA world hypothesis, according to which the emergence of translation was preceded by the era of self-replicating RNAs. Although such scenarios are less parsimonious from the evolutionary point of view, since they require simultaneous emergence and evolution of two classes of organic molecules, as well as the emergence of synchronized replication and translation, their major advantage is that they explain the development of processive and much more accurate protein-dependent replication.

Arginine multivalency stabilizes protein/RNA condensates

Biomolecular condensates assembled through liquid-liquid phase separation (LLPS) of proteins and RNAs are currently recognized to play an important role in cellular organization. Their assembly depends on the formation of a network of transient, multivalent interactions between flexible scaffold biomolecules. Understanding how protein and RNA sequences determine these interactions and ultimately regulate the phase separation is an open key challenge. Recent in vitro studies have revealed that arginine and lysine residues, which are enriched in most cellular condensates, have markedly distinct propensities to drive the LLPS of protein/RNA mixtures. Here, we employ explicit-solvent atomistic molecular dynamics simulations to shed light on the microscopic origin of this difference by investigating mixtures of polyU oligonucleotides with either polyR/polyK peptides. In agreement with experiments, our simulations indicate that arginine has a higher affinity for polyU than lysine both in highly diluted conditions and in concentrated solutions with a biomolecular density comparable to cellular condensate. The analysis of intermolecular contacts suggests that this differential behavior is due to the propensity of arginine side chains to simultaneously form a higher number of specific interactions with oligonucleotides, including hydrogen bonds and stacking interactions. Our results provide a molecular description of how the multivalency of the guanidinium group enables the coordination of multiple RNA groups by a single arginine residue, thus ultimately stabilizing protein/RNA condensates.

Frameshifting preserves key physicochemical properties of proteins

Frameshifts in protein coding sequences are widely perceived as resulting in either nonfunctional or even deleterious protein products. Indeed, frameshifts typically lead to markedly altered protein sequences and premature stop codons. By analyzing complete proteomes from all three domains of life, we demonstrate that, in contrast, several key physicochemical properties of protein sequences exhibit significant robustness against +1 and -1 frameshifts. In particular, we show that hydrophobicity profiles of many protein sequences remain largely invariant upon frameshifting. For example, over 2,900 human proteins exhibit a Pearson's correlation coefficient R between the hydrophobicity profiles of the original and the +1-frameshifted variants greater than 0.7, despite an average sequence identity between the two of only 6.5% in this group. We observe a similar effect for protein sequence profiles of affinity for certain nucleobases as well as protein sequence profiles of intrinsic disorder. Finally, analysis of significance and optimality demonstrates that frameshift stability is embedded in the structure of the universal genetic code and may have contributed to shaping it. Our results suggest that frameshifting may be a powerful evolutionary mechanism for creating new proteins with vastly different sequences, yet similar physicochemical properties to the proteins from which they originate.

Amino Acid Stabilization of Nucleic Acid Secondary Structure: Kinetic Insights from Single-Molecule Studies

Amino acid and nucleic acid interactions are central in biology and may have played a role in the evolutionary development of protein-based life from an early "RNA Universe." To explore the possible role of single amino acids in promoting nucleic acid folding, single-molecule Förster resonance energy transfer experiments have been implemented with a DNA hairpin construct (7 nucleotide double strand with a 40A loop) as a simple model for secondary structure formation. Exposure to positively charged amino acids (arginine and lysine) is found to clearly stabilize the secondary structure. Kinetically, each amino acid promotes folding by generating a large increase in the folding rate with little change in the unfolding rate. From analysis as a function of temperature, arginine and lysine are found to significantly increase the overall exothermicity of folding while imposing only a small entropic penalty on the folding process. Detailed investigations into the kinetics and thermodynamics of this amino acid-induced folding stability reveal arginine and lysine to interact with nucleic acids in a manner reminiscent of monovalent cations. Specifically, these observations are interpreted in the context of an ion atmosphere surrounding the nucleic acid, in which amino acid salts stabilize folding qualitatively like small monovalent cations but also exhibit differences because of the composition of their side chains.

RNA-protein interactions in an unstructured context

Despite their importance, our understanding of noncovalent RNA-protein interactions is incomplete. This especially concerns the binding between RNA and unstructured protein regions, a widespread class of such interactions. Here, we review the recent experimental and computational work on RNA-protein interactions in an unstructured context with a particular focus on how such interactions may be shaped by the intrinsic interaction affinities between individual nucleobases and protein side chains. Specifically, we articulate the claim that the universal genetic code reflects the binding specificity between nucleobases and protein side chains and that, in turn, the code may be seen as the Rosetta stone for understanding RNA-protein interactions in general.

mRNA/protein sequence complementarity and its determinants: The impact of affinity scales

It has recently been demonstrated that the nucleobase-density profiles of mRNA coding sequences are related in a complementary manner to the nucleobase-affinity profiles of their cognate protein sequences. Based on this, it has been proposed that cognate mRNA/protein pairs may bind in a co-aligned manner, especially if unstructured. Here, we study the dependence of mRNA/protein sequence complementarity on the properties of the nucleobase/amino-acid affinity scales used. Specifically, we sample the space of randomly generated scales by employing a Monte Carlo strategy with a fitness function that depends directly on the level of complementarity. For model organisms representing all three domains of life, we show that even short searches reproducibly converge upon highly optimized scales, implying that the topology of the underlying fitness landscape is decidedly funnel-like. Furthermore, the optimized scales, generated without any consideration of the physicochemical attributes of nucleobases or amino acids, resemble closely the nucleobase/amino-acid binding affinity scales obtained from experimental structures of RNA-protein complexes. This provides support for the claim that mRNA/protein sequence complementarity may indeed be related to binding between the two. Finally, we characterize suboptimal scales and show that intermediate-to-high complementarity can be reached by substantially diverse scales, but with select amino acids contributing disproportionally. Our results expose the dependence of cognate mRNA/protein sequence complementarity on the properties of the underlying nucleobase/amino-acid affinity scales and provide quantitative constraints that any physical scales need to satisfy for the complementarity to hold.

Messenger RNAs and proteins, two essential types of biopolymers, have recently been shown to exhibit closely related, complementary physicochemical properties. Specifically, density profiles of certain groups in messenger RNA sequences directly match the affinity profiles for precisely those groups in protein sequences they encode. Based on this, it has been suggested that these molecules may interact with each other specifically and in a co-aligned fashion, especially when unstructured. Here, we explore different amino-acid scales used in the above analysis to assess which of their properties dictate the observed matching. Specifically, we define the constraints that need to be satisfied by physical scales for the complementarity to hold and show that the previously derived nucleobase/amino-acid affinity scales indeed satisfy these constraints. As a whole, our work provides a quantitative foundation for understanding the putative messenger RNA/protein complementarity with implications in different areas of RNA/protein biology including transcription, translation, splicing and viral assembly.

Crowding in Cellular Environments at an Atomistic Level from Computer Simulations

The effects of crowding in biological environments on biomolecular structure, dynamics, and function remain not well understood. Computer simulations of atomistic models of concentrated peptide and protein systems at different levels of complexity are beginning to provide new insights. Crowding, weak interactions with other macromolecules and metabolites, and altered solvent properties within cellular environments appear to remodel the energy landscape of peptides and proteins in significant ways including the possibility of native state destabilization. Crowding is also seen to affect dynamic properties, both conformational dynamics and diffusional properties of macromolecules. Recent simulations that address these questions are reviewed here and discussed in the context of relevant experiments.

Cross-species conservation of complementary amino acid-ribonucleobase interactions and their potential for ribosome-free encoding

The role of amino acid-RNA nucleobase interactions in the evolution of RNA translation and protein-mRNA autoregulation remains an open area of research. We describe the inference of pairwise amino acid-RNA nucleobase interaction preferences using structural data from known RNA-protein complexes. We observed significant matching between an amino acid’s nucleobase affinity and corresponding codon content in both the standard genetic code and mitochondrial variants. Furthermore, we showed that knowledge of nucleobase preferences allows statistically significant prediction of protein primary sequence from mRNA using purely physiochemical information. Interestingly, ribosomal primary sequences were more accurately predicted than non-ribosomal sequences, suggesting a potential role for direct amino acid-nucleobase interactions in the genesis of amino acid-based ribosomal components. Finally, we observed matching between amino acid-nucleobase affinities and corresponding mRNA sequences in 35 evolutionarily diverse proteomes. We believe these results have important implications for the study of the evolutionary origins of the genetic code and protein-mRNA cross-regulation.

Interaction preferences between nucleobase mimetics and amino acids in aqueous solutions

Despite the paramount importance of protein-nucleic acid interactions in different cellular processes, our understanding of such interactions at the atomistic level remains incomplete. We have used molecular dynamics (MD) simulations and 15 μs of sampling time to study the behavior of amino acids and amino-acid sidechain analogs in aqueous solutions of different mimetics of naturally occurring nucleobases, including dimethylpyridine (DMP) and unsubstituted purine and pyrimidine rings. By using structural and energetic analysis, we have derived preference scales for the interaction of amino acids and their sidechain analogs with different nucleobase mimetics and have exhaustively compared them with each other. A close correspondence with a standard hydrophobicity measure in the case of the pyrimidine mimetic DMP and purines suggests that the hydrophobic effect is the main defining factor behind such interactions. We analyze our findings in the context of the origin of the genetic code and the recently proposed cognate mRNA-protein complementarity hypothesis. Most importantly, we show that unsubstituted purine and pyrimidine rings alone cannot differentiate between predominantly purine- and pyrimidine-coded amino acids, suggesting that for such specificity to exist, it must primarily reside in ring substituents.

Malleable nature of mRNA-protein compositional complementarity and its functional significance

It has recently been demonstrated that nucleobase-density profiles of typical mRNA coding sequences exhibit a complementary relationship with nucleobase-interaction propensity profiles of their cognate protein sequences. This finding supports the idea that the genetic code developed in response to direct binding interactions between amino acids and appropriate nucleobases, but also suggests that present-day mRNAs and their cognate proteins may be physicochemically complementary to each other and bind. Here, we computationally recode complete Methanocaldococcus jannaschii, Escherichia coli and Homo sapiens mRNA transcriptomes and analyze how much complementary matching of synonymous mRNAs can vary, while keeping protein sequences fixed. We show that for most proteins there exist cognate mRNAs that improve, but also significantly worsen the level of native matching (e.g. by 1.8 viz. 7.6 standard deviations on average for H. sapiens, respectively), with the least malleable proteins in this sense being strongly enriched in nuclear localization and DNA-binding functions. Even so, we show that the majority of recodings for most proteins result in pronounced complementarity. Our results suggest that the genetic code was designed for favorable, yet tunable compositional complementarity between mRNAs and their cognate proteins, supporting the hypothesis that the interactions between the two were an important defining element behind the code's origin.