Crystal structure of a domain-swapped photoactivatable sfGFP variant provides evidence for GFP folding pathway

Genetically Encoded Photocaged Proteinogenic and Non-Proteinogenic Amino Acids

Photocaged amino acids could be genetically encoded into proteins via genetic code expansion (GCE) and constitute unique tools for innovative protein engineering. There are a number of photocaged proteinogenic amino acids that allow strategic conversion of proteins into their photocaged variants, thus enabling spatiotemporal and non-invasive regulation of protein functions using light. Meanwhile, there are a hand of photocaged non-proteinogenic amino acids that address the challenges in directly encoding certain non-canonical amino acids (ncAAs) that structurally resemble proteinogenic ones or possess highly reactive functional groups. Herein, we would like to summarize the efforts in encoding photocaged proteinogenic and non-proteinogenic amino acids, hoping to draw more attention to this fruitful and exciting scientific campaign.

Regulation of IL-24/IL-20R2 complex formation using photocaged tyrosines and UV light

Human interleukin 24 (IL-24) is a multifunctional cytokine that represents an important target for autoimmune diseases and cancer. Since the biological functions of IL-24 depend on interactions with membrane receptors, on-demand regulation of the affinity between IL-24 and its cognate partners offers exciting possibilities in basic research and may have applications in therapy. As a proof-of-concept, we developed a strategy based on recombinant soluble protein variants and genetic code expansion technology to photocontrol the binding between IL-24 and one of its receptors, IL-20R2. Screening of non-canonical ortho-nitrobenzyl-tyrosine (NBY) residues introduced at several positions in both partners was done by a combination of biophysical and cell signaling assays. We identified one position for installing NBY, tyrosine70 of IL-20R2, which results in clear impairment of heterocomplex assembly in the dark. Irradiation with 365-nm light leads to decaging and reconstitutes the native tyrosine of the receptor that can then associate with IL-24. Photocaged IL-20R2 may be useful for the spatiotemporal control of the JAK/STAT phosphorylation cascade.

Best practices for time-resolved serial synchrotron crystallography

With recent developments in X-ray sources, instrumentation and data-analysis tools, time-resolved crystallographic experiments, which were originally the preserve of a few expert groups, are becoming simpler and can be carried out at more radiation sources, and are thus increasingly accessible to a growing user base. However, these experiments are just that: discrete experiments, not just `data collections'. As such, careful planning and consideration of potential pitfalls is required to enable a successful experiment. Here, some of the key factors that should be considered during the planning and execution of a time-resolved structural study are outlined, with a particular focus on synchrotron-based experiments.

Why Are Gly31, Gly33, and Gly35 Highly Conserved in All Fluorescent Proteins?

Green fluorescent protein (GFP)-like fluorescent proteins have been found in more than 120 species. Although the proteins have little sequence identity, Gly31, 33, and 35 are 87, 100, and 95% conserved across all species, respectively. All GFP-like proteins have a β-barrel structure composed of 11 β-sheets, and the 3 conserved glycines are located in the second β-sheet. Molecular dynamics (MD) simulations have shown that mutating one or more of the glycines to alanines most likely does not reduce chromophore formation in correctly folded immature fluorescent proteins. MD and protein characterization of alanine mutants indicate that mutation of the conserved glycines leads to misfolding. Gly31, 33, and 35 are essential to maintain the integrity of the β_1-3 triad that is the last structural element to slot in place in the formation of the canonical fluorescent protein β-barrel. Glycines located in β-sheets may have a similar role in the formation of other non-GFP β-barrels.

Protein Secondary Structure Affects Glycan Clustering in Native Mass Spectrometry

Infection by the humannoroviruses (hNoV), for the vast majority of strains, requires attachment of the viral capsid to histo blood group antigens (HBGAs). The HBGA-binding pocket is formed by dimers of the protruding domain (P dimers) of the capsid protein VP1. Several studies have focused on HBGA binding to P dimers, reporting binding affinities and stoichiometries. However, nuclear magnetic resonance spectroscopy (NMR) and native mass spectrometry (MS) analyses yielded incongruent dissociation constants (K_D) for the binding of HBGAs to P dimers and, in some cases, disagreed on whether glycans bind at all. We hypothesized that glycan clustering during electrospray ionization in native MS critically depends on the physicochemical properties of the protein studied. It follows that the choice of a reference protein is crucial. We analysed carbohydrate clustering using various P dimers and eight non-glycan binding proteins serving as possible references. Data from native and ion mobility MS indicate that the mass fraction of β-sheets has a strong influence on the degree of glycan clustering. Therefore, the determination of specific glycan binding affinities from native MS must be interpreted cautiously.