Human adenovirus 52 (HAdV-52) is one of only three known HAdVs equipped with both a long and a short fiber protein. While the long fiber binds to the coxsackie and adenovirus receptor, the function of the short fiber in the virus life cycle is poorly understood. Here, we show, by glycan microarray analysis and cellular studies, that the short fiber knob (SFK) of HAdV-52 recognizes long chains of α-2,8-linked polysialic acid (polySia), a large posttranslational modification of selected carrier proteins, and that HAdV-52 can use polySia as a receptor on target cells. X-ray crystallography, NMR, molecular dynamics simulation, and structure-guided mutagenesis of the SFK reveal that the nonreducing, terminal sialic acid of polySia engages the protein with direct contacts, and that specificity for polySia is achieved through subtle, transient electrostatic interactions with additional sialic acid residues. In this study, we present a previously unrecognized role for polySia as a cellular receptor for a human viral pathogen. Our detailed analysis of the determinants of specificity for this interaction has general implications for protein–carbohydrate interactions, particularly concerning highly charged glycan structures, and provides interesting dimensions on the biology and evolution of members of Human mastadenovirus G. Significance We present here that adenovirus type 52 (HAdV-52) attaches to target cells through a mechanism not previously observed in other human pathogenic viruses. The interaction involves unusual, transient, electrostatic interactions between the short fiber capsid protein and polysialic acid (polySia)-containing receptors on target cells. Knowledge about the binding interactions between polySia and its natural ligands is relatively limited, and our results therefore provide additional insight not only into adenovirus biology but also into the structural basis of polySia function. Since polySia can be found in high expression levels in brain and lung cancers where its presence is associated with poor prognosis, we suggest that this polySia-binding adenovirus could be useful for design of vectors for gene therapy of these cancers.

Most adenoviruses attach to host cells by means of the protruding fiber protein that binds to host cells via the coxsackievirus and adenovirus receptor (CAR) protein. Human adenovirus type 52 (HAdV-52) is one of only three gastroenteritis-causing HAdVs that are equipped with two different fiber proteins, one long and one short. Here we show, by means of virion-cell binding and infection experiments, that HAdV-52 can also attach to host cells via CAR, but most of the binding depends on sialylated glycoproteins. Glycan microarray, flow cytometry, surface plasmon resonance and ELISA analyses reveal that the terminal knob domain of the long fiber (52LFK) binds to CAR, and the knob domain of the short fiber (52SFK) binds to sialylated glycoproteins. X-ray crystallographic analysis of 52SFK in complex with 2-O-methylated sialic acid combined with functional studies of knob mutants revealed a new sialic acid binding site compared to other, known adenovirus:glycan interactions. Our findings shed light on adenovirus biology and may help to improve targeting of adenovirus-based vectors for gene therapy. HAdVs are common pathogens in humans, causing disease mainly in eyes, airways and gastrointestinal tract. Most HAdVs are equipped with twelve protruding fiber proteins that mediate attachment to host cell receptor molecules. Recently, a new human gastroenteritis-associated adenovirus (HAdV-52) was identified and classified as the first member of a novel species (HAdV-G). Unlike most other HAdVs, this virus contains two different fiber proteins, a long and a short one, a feature shared only with the two members of species HAdV-F (HAdV-40 and -41). To gain further insights into the mechanisms of HAdV-52 infection of human cells, we set out to identify the host cell receptors used by the long and short fibers. We find that the long fiber binds to a protein-based receptor known as the coxsackievirus and adenovirus receptor (CAR), and that the short fiber binds to glycoproteins that contain sialic acid-capped glycans. The crystal structure determination of a complex of the short fiber knob bound to sialic acid demonstrates that this interaction is unique among HAdVs, and bioinformatic analysis indicates that simian AdVs may also engage sialic acids in the manner seen in HAdV-52. The results presented here provide insights into the plasticity of adenovirus-host cell interactions.

Chikungunya virus (CHIKV) is a re-emerging, mosquito-transmitted, enveloped positive stranded RNA virus. Chikungunya fever is characterized by acute and chronic debilitating arthritis. Although multiple host factors have been shown to enhance CHIKV infection, the molecular mechanisms of cell entry and entry factors remain poorly understood. The phosphatidylserine-dependent receptors, T-cell immunoglobulin and mucin domain 1 (TIM-1) and Axl receptor tyrosine kinase (Axl), are transmembrane proteins that can serve as entry factors for enveloped viruses. Previous studies used pseudoviruses to delineate the role of TIM-1 and Axl in CHIKV entry. Conversely, here, we use the authentic CHIKV and cells ectopically expressing TIM-1 or Axl and demonstrate a role for TIM-1 in CHIKV infection. To further characterize TIM-1-dependent CHIKV infection, we generated cells expressing domain mutants of TIM-1. We show that point mutations in the phosphatidylserine binding site of TIM-1 lead to reduced cell binding, entry, and infection of CHIKV. Ectopic expression of TIM-1 renders immortalized keratinocytes permissive to CHIKV, whereas silencing of endogenously expressed TIM-1 in human hepatoma cells reduces CHIKV infection. Altogether, our findings indicate that, unlike Axl, TIM-1 readily promotes the productive entry of authentic CHIKV into target cells.

A cryo-EM structure reveals unique traits of a human adenovirus that has acquired gastrointestinal tropism.

Most adenoviruses bind directly to the coxsackie and adenovirus receptor (CAR) on target cells in vitro, but recent research has shown that adenoviruses can also use soluble components in body fluids for indirect binding to target cells. These mechanisms have been identified upon addressing the questions of how to de- and retarget adenovirus-based vectors for human gene and cancer therapy, but the newly identified mechanisms also suggest that the role of body fluids and their components may also be of importance for natural, primary infections. Here we demonstrate that plasma, saliva, and tear fluid promote binding and infection of adenovirus type 5 (Ad5) in respiratory and ocular epithelial cells, which corresponds to the natural tropism of most adenoviruses, and that plasma promotes infection by Ad31. By using a set of binding and infection experiments, we also found that Ad5 and Ad31 require coagulation factors IX (FIX) or X (FX) or just FIX, respectively, for efficient binding and infection. The concentrations of these factors that were required for maximum binding were 1/100th of the physiological concentrations. Preincubation of virions with heparin or pretreatment of cells with heparinase I indicated that the role of cell surface heparan sulfate during FIX- and FX-mediated adenovirus binding and infection is mechanistically serotype specific. We conclude that the use of coagulation factors by adenoviruses may be of importance not only for the liver tropism seen when administering adenovirus vectors to the circulation but also during primary infections by wild-type viruses of their natural target cell types.

Identification of host factors affecting individual SARS-CoV-2 susceptibility will provide a better understanding of the large variations in disease severity and will identify potential factors that can be used, or targeted, in antiviral drug development. With the use of an advanced lung cell model established from several human donors, we identified cellular protease inhibitors, serpins, as host factors that restrict SARS-CoV-2 infection.