Altered receptor binding, antibody evasion and retention of T cell recognition by the SARS-CoV-2 XBB.1.5 spike protein

The XBB.1.5 variant of SARS-CoV-2 has rapidly achieved global dominance and exhibits a high growth advantage over previous variants. Preliminary reports suggest that the success of XBB.1.5 stems from mutations within its spike glycoprotein, causing immune evasion and enhanced receptor binding. We present receptor binding studies that demonstrate retention of binding contacts with the human ACE2 receptor and a striking decrease in binding to mouse ACE2 due to the revertant R493Q mutation. Despite extensive evasion of antibody binding, we highlight a region on the XBB.1.5 spike protein receptor binding domain (RBD) that is recognized by serum antibodies from a donor with hybrid immunity, collected prior to the emergence of the XBB.1.5 variant. T cell assays reveal high frequencies of XBB.1.5 spike-specific CD4+ and CD8+ T cells amongst donors with hybrid immunity, with the CD4+ T cells skewed towards a Th1 cell phenotype and having attenuated effector cytokine secretion as compared to ancestral spike protein-specific cells. Thus, while the XBB.1.5 variant has retained efficient human receptor binding and gained antigenic alterations, it remains susceptible to recognition by T cells induced via vaccination and previous infection.

Allosteric modulation of ryanodine receptor RyR1 by nucleotide derivatives

The coordinated release of Ca2+ from the sarcoplasmic reticulum (SR) is critical for excitation-contraction coupling. This release is facilitated by ryanodine receptors (RyRs) that are embedded in the SR membrane. In skeletal muscle, activity of RyR1 is regulated by metabolites such as ATP, which upon binding increase channel open probability (Po). To obtain structural insights into the mechanism of RyR1 priming by ATP, we determined several cryo-EM structures of RyR1 bound individually to ATP-γ-S, ADP, AMP, adenosine, adenine, and cAMP. We demonstrate that adenine and adenosine bind RyR1, but AMP is the smallest ATP derivative capable of inducing long-range (>170 Å) structural rearrangements associated with channel activation, establishing a structural basis for key binding site interactions that are the threshold for triggering quaternary structural changes. Our finding that cAMP also induces these structural changes and results in increased channel opening suggests its potential role as an endogenous modulator of RyR1 conductance.

Structural analysis of receptor engagement and antigenic drift within the BA.2 spike protein

The BA.2 sub-lineage of the Omicron (B.1.1.529) severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant rapidly supplanted the original BA.1 sub-lineage in early 2022. Both lineages threatened the efficacy of vaccine-elicited antibodies and acquired increased binding to several mammalian ACE2 receptors. Cryoelectron microscopy (cryo-EM) analysis of the BA.2 spike (S) glycoprotein in complex with mouse ACE2 (mACE2) identifies BA.1- and BA.2-mutated residues Q493R, N501Y, and Y505H as complementing non-conserved residues between human and mouse ACE2, rationalizing the enhanced S protein-mACE2 interaction for Omicron variants. Cryo-EM structures of the BA.2 S-human ACE2 complex and of the extensively mutated BA.2 amino-terminal domain (NTD) reveal a dramatic reorganization of the highly antigenic N1 loop into a β-strand, providing an explanation for decreased binding of the BA.2 S protein to antibodies isolated from BA.1-convalescent patients. Our analysis reveals structural mechanisms underlying the antigenic drift in the rapidly evolving Omicron variant landscape.

SARS-CoV-2 Omicron variant: Antibody evasion and cryo-EM structure of spike protein-ACE2 complex

The newly reported Omicron variant is poised to replace Delta as the most prevalent SARS-CoV-2 variant across the world. Cryo-EM structural analysis of the Omicron variant spike protein in complex with human ACE2 reveals new salt bridges and hydrogen bonds formed by mutated residues R493, S496 and R498 in the RBD with ACE2. These interactions appear to compensate for other Omicron mutations such as K417N known to reduce ACE2 binding affinity, resulting in similar biochemical ACE2 binding affinities for Delta and Omicron variants. Neutralization assays show that pseudoviruses displaying the Omicron spike protein exhibit increased antibody evasion. The increase in antibody evasion, together with retention of strong interactions at the ACE2 interface, thus represent important molecular features that likely contribute to the rapid spread of the Omicron variant.

SARS-CoV-2 Omicron variant: Antibody evasion and cryo-EM structure of spike protein-ACE2 complex

[Figure: see text].

Cryo-electron microscopy structures of the N501Y SARS-CoV-2 spike protein in complex with ACE2 and 2 potent neutralizing antibodies

The recently reported "UK variant" (B.1.1.7) of SARS-CoV-2 is thought to be more infectious than previously circulating strains as a result of several changes, including the N501Y mutation. We present a 2.9-Å resolution cryo-electron microscopy (cryo-EM) structure of the complex between the ACE2 receptor and N501Y spike protein ectodomains that shows Y501 inserted into a cavity at the binding interface near Y41 of ACE2. This additional interaction provides a structural explanation for the increased ACE2 affinity of the N501Y mutant, and likely contributes to its increased infectivity. However, this mutation does not result in large structural changes, enabling important neutralization epitopes to be retained in the spike receptor binding domain. We confirmed this through biophysical assays and by determining cryo-EM structures of spike protein ectodomains bound to 2 representative potent neutralizing antibody fragments.

ORF Capture-Seq as a versatile method for targeted identification of full-length isoforms

Most human protein-coding genes are expressed as multiple isoforms, which greatly expands the functional repertoire of the encoded proteome. While at least one reliable open reading frame (ORF) model has been assigned for every coding gene, the majority of alternative isoforms remains uncharacterized due to (i) vast differences of overall levels between different isoforms expressed from common genes, and (ii) the difficulty of obtaining full-length transcript sequences. Here, we present ORF Capture-Seq (OCS), a flexible method that addresses both challenges for targeted full-length isoform sequencing applications using collections of cloned ORFs as probes. As a proof-of-concept, we show that an OCS pipeline focused on genes coding for transcription factors increases isoform detection by an order of magnitude when compared to unenriched samples. In short, OCS enables rapid discovery of isoforms from custom-selected genes and will accelerate mapping of the human transcriptome.