Strategies for rapid production of crystallization quality coatomer WD40 domains

Sequences in the Cytoplasmic Tail Contribute to the Intracellular Trafficking and the Cell Surface Localization of SARS-CoV-2 Spike Protein

Spike protein is a surface glycoprotein of the SARS-CoV-2 coronavirus, providing interaction of the coronavirus with angiotensin-converting enzyme 2 (ACE2) on the host cell. The cytoplasmic tail of the S protein plays an important role in an intracellular transport and translocation of the glycoprotein to the plasma membrane. The cytoplasmic domain of the S protein contains binding sites for COPI, COPII, and SNX27, which are required for the intracellular trafficking of this glycoprotein. In addition, the cytoplasmic domain of the S protein contains S-palmitoylation sites. S-palmitoylation increases the hydrophobicity of the S protein by regulating its transport to the plasma membrane. The cytoplasmic tail of the S protein has a signaling sequence that provides interaction with the ERM family proteins, which may mediate communication between the cell membrane and the actin cytoskeleton. This review examines the role of the cytoplasmic tail of the SARS-CoV-2 S protein in its intracellular transport and translocation to the plasma membrane. Understanding these processes is necessary not only for the development of vaccines based on mRNA or adenovirus vectors encoding the full-length spike (S) protein, but also for the therapy of the new coronavirus infection (COVID-19).

Recent Advances in Expression Screening and Sample Evaluation for Structural Studies of Membrane Proteins

Membrane proteins are involved in numerous biological processes and represent more than half of all drug targets; thus, structural information on these proteins is invaluable. However, the low expression level of membrane proteins, as well as their poor stability in solution and tendency to precipitate and aggregate, are major bottlenecks in the preparation of purified membrane proteins for structural studies. Traditionally, the evaluation of membrane protein constructs for structural studies has been quite time consuming and expensive since it is necessary to express and purify the proteins on a large scale, particularly for X-ray crystallography. The emergence of fluorescence detection size exclusion chromatography (FSEC) has drastically changed this situation, as this method can be used to rapidly evaluate the expression and behavior of membrane proteins on a small scale without the need for purification. FSEC has become the most widely used method for the screening of expression conditions and sample evaluation for membrane proteins, leading to the successful determination of numerous structures. Even in the era of cryo-EM, FSEC and the new generation of FSEC derivative methods are being widely used in various manners to facilitate structural analysis. In addition, the application of FSEC is not limited to structural analysis; this method is also widely used for functional analysis of membrane proteins, including for analysis of oligomerization state, screening of antibodies and ligands, and affinity profiling. This review presents the latest advances and applications in membrane protein expression screening and sample evaluation, with a particular focus on FSEC methods.

A single C-terminal residue controls SARS-CoV-2 spike trafficking and incorporation into VLPs

The spike (S) protein of SARS-CoV-2 is delivered to the virion assembly site in the ER-Golgi Intermediate Compartment (ERGIC) from both the ER and cis-Golgi in infected cells. However, the relevance and modulatory mechanism of this bidirectional trafficking are unclear. Here, using structure-function analyses, we show that S incorporation into virus-like particles (VLP) and VLP fusogenicity are determined by coatomer-dependent S delivery from the cis-Golgi and restricted by S-coatomer dissociation. Although S mimicry of the host coatomer-binding dibasic motif ensures retrograde trafficking to the ERGIC, avoidance of the host-like C-terminal acidic residue is critical for S-coatomer dissociation and therefore incorporation into virions or export for cell-cell fusion. Because this C-terminal residue is the key determinant of SARS-CoV-2 assembly and fusogenicity, our work provides a framework for the export of S protein encoded in genetic vaccines for surface display and immune activation.