Dolan, B. P. | bioRxiv Science

bioRxiv2022

NEDD8 activity is important for direct antigen MHC class I antigen presentation

Successful direct MHC class I antigen presentation is dependent on the protein degradation machinery of the cell to generate antigenic peptides which can be loaded onto MHC class I molecules for surveillance by CD8+ T cells of the immune system. Most often this process involves the ubiquitin-proteasome system, however other ubiquitin-like (UBL) proteins have also been implicated in protein degradation and direct antigen presentation. Here, we examine the role of neuronal precursor cell-expressed developmentally down-regulated protein 8 (NEDD8) in direct antigen presentation. NEDD8 is the UBL with highest similarity to ubiquitin and fusion of NEDD8 to the amino-terminus of a target protein can lead to the target proteins degradation. We find that appending NEDD8 to the N-terminus of the model antigen ovalbumin resulted in degradation by both the proteasome and autophagy protein degradation pathways, but only proteasomal degradation, involving the proteasomal subunit NEDD8 ultimate buster 1 (NUB1), resulted in peptide presentation. When directly compare to ubiquitin, NEDD8-fusion was less efficient at generating peptides. However, inactivation of the NEDD8-conugation machinery by treating cells with MLN4924, inhibited the presentation of peptides from Defective Ribosomal Products (DRiPs) derived from a model antigen. These results demonstrate that NEDD8 activity in the cell is important for direct antigen presentation, but not by directly targeting proteins for degradation.

immunology↗

bioRxiv2021

Variations in cell-surface ACE2 levels alter direct binding of SARS-CoV-2 Spike protein and viral infectivity: Implications for measuring Spike protein interactions with animal ACE2 orthologs

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of COVID-19, the most severe pandemic in a century. The virus gains access to host cells when the viral Spike protein (S-protein) binds to the host cell-surface receptor angiotensin-converting enzyme 2 (ACE2). Studies have attempted to understand SARS-CoV-2 S-protein interaction with vertebrate orthologs of ACE2 by expressing ACE2 orthologs in mammalian cells and measuring viral infection or S-protein binding. Often these cells only transiently express ACE2 proteins and levels of ACE2 at the cell surface are not quantified. Here, we describe a cell-based assay that uses stably transfected cells expressing ACE2 proteins in a bi-cistronic vector with an easy to quantify reporter protein to normalize ACE2 expression. We found that both binding of the S-protein receptor-binding domain (RBD) and infection with a SARS-CoV-2 pseudovirus is proportional to the amount of human ACE2 expressed at the cell surface, which can be inferred by quantifying the level of reporter protein, Thy1.1. We also compared different ACE2 orthologs which were expressed in stably transfected cells expressing equivalent levels of Thy1.1. When ranked for either viral infectivity or RBD binding, mouse ACE2 had a weak to undetectable affinity for S-protein while human ACE2 was the highest level detected and feline ACE2 had an intermediate phenotype. The generation of stably transfected cells whose ACE2 level can be normalized for cross-ortholog comparisons allows us to create a reusable cellular library useful for measuring emerging SARS-CoV-2 variants ability to potentially infect different animals. ImportanceSARS-CoV-2 is a zoonotic virus responsible for the worst global pandemic in a century. An understanding of how the virus can infect other vertebrate species is important for controlling viral spread and understanding the natural history of the virus. Here we describe a method to generate cells stably expressing equivalent levels of different ACE2 orthologs, the receptor for SARS-CoV-2, on the surface of a human cell line. We find that both binding of the viral Spike protein receptor binding domain (RBD) and infection of cells with a SARS-CoV-2 pseudovirus are proportional to ACE2 levels at the cell surface. Adaptation of this method will allow for the creation of a library of stable transfected cells expressing equivalent levels of different vertebrate ACE2 orthologs which can be repeatedly used for identifying vertebrate species which may be susceptible to infection with SARS-CoV-2 and its many variants.

microbiology↗