Modulating Polyalanine Motifs of Synthetic Spidroin for Controllable Preassembly and Strong Fiber Formation

Spider dragline (major ampullate) silk is one of the toughest known fibers in nature and exhibits an excellent combination of high tensile strength and elasticity. Increasing evidence has indicated that preassembly plays a crucial role in facilitating the proper assembly of silk fibers by bridging the mesoscale gap between spidroin molecules and the final strong fibers. However, it remains challenging to control the preassembly of spidroins and investigate its influence on fiber structural and mechanical properties. In this study, we explored to bridge this gap by modulating the polyalanine (polyA) motifs in repetitive region of spidroins to tune their preassemblies in aqueous dope solutions. Three biomimetic silk proteins with varying numbers of alanine residues in polyA motif and comparable molecular weights were designed and biosynthesized, termed as N16C-5A, N15C-8A, and N13C-12A, respectively. It was found that all three proteins could form nanofibril assemblies in the concentrated aqueous dopes, but the size and structural stability of the fibrils were distinct from each other. The silk protein N15C-8A with 8 alanine residues in polyA motif allowed for the formation of stable nanofibril assemblies with a length of approximately 200 nm, which were not prone to disassemble or aggregate as that of N16C-5A and N13C-12A. More interestingly, the stable fibril assembly of N15C-8A enabled spinning of simultaneously strong (623.3 MPa) and tough (107.1 MJ m-3) synthetic fibers with fine molecular orientation and close interface packing of fibril bundles. This work highlights that modulation of polyA motifs is a feasible way to tune the morphology and stability of the spidroin preassemblies in dope solutions, thus controlling the structural and mechanical properties of the resulting fibers.

Pre-organized landscape of T cell surface

T cell activation is initiated by the recognition of specific antigenic peptides and subsequently accomplished by complex signaling cascades. These aspects have been extensively studied for decades as pivotal factors in the establishment of adaptive immunity. However, how receptors or signaling molecules are organized in the resting state prior to encountering antigens has received less attention. Recent advancements in super-resolution microscopy techniques have revealed topographically controlled pre-formed organization of key molecules involved in antigen recognition and signal transduction on microvillar projections of T cells before activation and substantial effort has been dedicated to characterizing the topological structure of resting T cells over the past decade. This review will summarize our current understanding of how key surface receptors are pre-organized on the T-cell plasma membrane and discuss the potential role of these receptors, which are preassembled prior to ligand binding in the early activation events of T cells.

Evolution of Complexity. Molecular Aspects of Preassembly

An extension of neo-Darwinism, termed preassembly, states that genetic material required for many complex traits, such as echolocation, was present long before emergence of the traits. Assembly of genes and gene segments had occurred over protracted time-periods within large libraries of non-coding genes. Epigenetic factors ultimately promoted transfers from noncoding to coding genes, leading to abrupt formation of the trait via de novo genes. This preassembly model explains many observations that to this present day still puzzle biologists: formation of super-complexity in the absence of multiple fossil precursors, as with bat echolocation and flowering plants; major genetic and physical alterations occurring in just a few thousand years, as with housecat evolution; lack of precursors preceding lush periods of species expansion, as in the Cambrian explosion; and evolution of costly traits that exceed their need during evolutionary times, as with human intelligence. What follows in this paper is a mechanism that is not meant to supplant neo-Darwinism; instead, preassembly aims to supplement current ideas when complexity issues leave them struggling.

Electro-mediated PhotoRedox Catalysis for Selective C(sp3 )-O Cleavages of Phosphinated Alcohols to Carbanions

We report a novel example of electro-mediated photoredox catalysis (e-PRC) in the reductive cleavage of C(sp3 )-O bonds of phosphinated alcohols to alkyl carbanions. As well as deoxygenations, olefinations are reported which are E-selective and can be made Z-selective in a tandem reduction/photosensitization process where both steps are photoelectrochemically promoted. Spectroscopy, computation, and catalyst structural variations reveal that our new naphthalene monoimide-type catalyst allows for an intimate dispersive precomplexation of its radical anion form with the phosphinate substrate, facilitating a reactivity-determining C(sp3 )-O cleavage. Surprisingly and in contrast to previously reported photoexcited radical anion chemistries, our conditions tolerate aryl chlorides/bromides and do not give rise to Birch-type reductions.

An Alternative Molecular View of Evolution: How DNA was Altered over Geological Time

Four natural phenomena are cited for their defiance of conventional neo-Darwinian analysis: human intelligence; cat domesticity; the Cambrian explosion; and convergent evolution. 1. Humans are now far more intelligent than needed in their hunting–gathering days >10,000 years ago. 2. Domestic cats evolved from wildcats via major genetic and physical changes, all occurring in less than 12,000 years. 3. The Cambrian explosion refers to the remarkable expansion of species that mystifies evolutionists, as there is a total lack of fossil evidence for precursors of this abundant new life. 4. Convergent evolution often involves formation of complex, multigene traits in two or more species that have no common ancestor. These four evolutionary riddles are discussed in terms of a proposed “preassembly” mechanism in which genes and gene precursors are collected silently and randomly over extensive time periods within huge non-coding sections of DNA. This is followed by epigenetic release of the genes, when the environment so allows, and by natural selection. In neo-Darwinism, macroevolution of complex traits involves multiple mutation/selections, with each of the resulting intermediates being more favorable to the species than the previous one. Preassembly, in contrast, invokes natural selection only after a partially or fully formed trait is already in place. Preassembly does not supplant neo-Darwinism but, instead, supplements neo-Darwinism in those important instances where the classical theory is wanting.

Mutations in C11orf70 Cause Primary Ciliary Dyskinesia with Randomization of Left/Right Body Asymmetry Due to Defects of Outer and Inner Dynein Arms

Primary ciliary dyskinesia (PCD) is characterized by chronic airway disease, male infertility, and randomization of the left/right body axis as a result of defects of motile cilia and sperm flagella. We identified loss-of-function mutations in the open-reading frame C11orf70 in PCD individuals from five distinct families. Transmission electron microscopy analyses and high-resolution immunofluorescence microscopy demonstrate that loss-of-function mutations in C11orf70 cause immotility of respiratory cilia and sperm flagella, respectively, as a result of the loss of axonemal outer (ODAs) and inner dynein arms (IDAs), indicating that C11orf70 is involved in cytoplasmic assembly of dynein arms. Expression analyses of C11orf70 showed that C11orf70 is expressed in ciliated respiratory cells and that the expression of C11orf70 is upregulated during ciliogenesis, similar to other previously described cytoplasmic dynein-arm assembly factors. Furthermore, C11orf70 shows an interaction with cytoplasmic ODA/IDA assembly factor DNAAF2, supporting our hypothesis that C11orf70 is a preassembly factor involved in the pathogenesis of PCD. The identification of additional genetic defects that cause PCD and male infertility is of great importance for the clinic as well as for genetic counselling.

Establishment of the early cilia preassembly protein complex during motile ciliogenesis

Motile cilia are characterized by dynein motor units, which preassemble in the cytoplasm before trafficking into the cilia. Proteins required for dynein preassembly were discovered by finding human mutations that result in absent ciliary motors, but little is known about their expression, function, or interactions. By monitoring ciliogenesis in primary airway epithelial cells and MCIDAS-regulated induced pluripotent stem cells, we uncovered two phases of expression of preassembly proteins. An early phase, composed of HEATR2, SPAG1, and DNAAF2, preceded other preassembly proteins and was independent of MCIDAS regulation. The early preassembly proteins colocalized within perinuclear foci that also contained dynein arm proteins. These proteins also interacted based on immunoprecipitation and Förster resonance energy transfer (FRET) studies. FRET analysis of HEAT domain deletions and human mutations showed that HEATR2 interacted with itself and SPAG1 at multiple HEAT domains, while DNAAF2 interacted with SPAG1. Human mutations in HEATR2 did not affect this interaction, but triggered the formation of p62/Sequestosome-1-positive aggregates containing the early preassembly proteins, suggesting that degradation of an early preassembly complex is responsible for disease and pointing to key regions required for HEATR2 scaffold stability. We speculate that HEATR2 is an early scaffold for the initiation of dynein complex assembly in motile cilia.

The G protein Gi1 exhibits basal coupling but not preassembly with G protein-coupled receptors

The G_i/o protein family transduces signals from a diverse group of G protein-coupled receptors (GPCRs). The observed specificity of G_i/o-GPCR coupling and the high rate of G_i/o signal transduction have been hypothesized to be enabled by the existence of stable associates between G_i/o proteins and their cognate GPCRs in the inactive state (G_i/o-GPCR preassembly). To test this hypothesis, we applied the recently developed technique of two-photon polarization microscopy (2PPM) to Gα_i1 subunits labeled with fluorescent proteins and four GPCRs: the α_2A-adrenergic receptor, GABA_B, cannabinoid receptor type 1 (CB₁R), and dopamine receptor type 2. Our experiments with non-dissociating mutants of fluorescently labeled Gα_i1 subunits (exhibiting impaired dissociation from activated GPCRs) showed that 2PPM is capable of detecting GPCR-G protein interactions. 2PPM experiments with non-mutated fluorescently labeled Gα_i1 subunits and α_2A-adrenergic receptor, GABA_B, or dopamine receptor type 2 receptors did not reveal any interaction between the G_i1 protein and the non-stimulated GPCRs. In contrast, non-stimulated CB₁R exhibited an interaction with the G_i1 protein. Further experiments revealed that this interaction is caused solely by CB₁R basal activity; no preassembly between CB₁R and the G_i1 protein could be observed. Our results demonstrate that four diverse GPCRs do not preassemble with non-active G_i1 However, we also show that basal GPCR activity allows interactions between non-stimulated GPCRs and G_i1 (basal coupling). These findings suggest that G_i1 interacts only with active GPCRs and that the well known high speed of GPCR signal transduction does not require preassembly between G proteins and GPCRs.

Ghrelin receptor conformational dynamics regulate the transition from a preassembled to an active receptor:Gq complex

How G protein-coupled receptor conformational dynamics control G protein coupling to trigger signaling is a key but still open question. We addressed this question with a model system composed of the purified ghrelin receptor assembled into lipid discs. Combining receptor labeling through genetic incorporation of unnatural amino acids, lanthanide resonance energy transfer, and normal mode analyses, we directly demonstrate the occurrence of two distinct receptor:Gq assemblies with different geometries whose relative populations parallel the activation state of the receptor. The first of these assemblies is a preassembled complex with the receptor in its basal conformation. This complex is specific of Gq and is not observed with Gi. The second one is an active assembly in which the receptor in its active conformation triggers G protein activation. The active complex is present even in the absence of agonist, in a direct relationship with the high constitutive activity of the ghrelin receptor. These data provide direct evidence of a mechanism for ghrelin receptor-mediated Gq signaling in which transition of the receptor from an inactive to an active conformation is accompanied by a rearrangement of a preassembled receptor:G protein complex, ultimately leading to G protein activation and signaling.

Receptor-G protein interaction studied by bioluminescence resonance energy transfer: lessons from protease-activated receptor 1

Since its development, the bioluminescence resonance energy transfer (BRET) approach has been extensively applied to study G protein-coupled receptors (GPCRs) in real-time and in live cells. One of the major aspects of GPCRs investigated in considerable details is their physical coupling to the heterotrimeric G proteins. As a result, new concepts have emerged, but few questions are still a matter of debate illustrating the complexity of GPCR-G protein interactions and coupling. Here, we summarized the recent advances on our understanding of GPCR-G protein coupling based on BRET approaches and supported by other FRET-based studies. We essentially focused on our recent studies in which we addressed the concept of preassembly vs. the agonist-dependent interaction between the protease-activated receptor 1 (PAR1) and its cognate G proteins. We discussed the concept of agonist-induced conformational changes within the preassembled PAR1-G protein complexes as well as the critical question how the multiple coupling of PAR1 with two different G proteins, Gαi1 and Gα12, but also β-arrestin 1, can be regulated.