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Abstract The Symbol Nomenclature for Glycans (SNFG) is a community-curated standard for the depiction of monosaccharides and complex glycans using various colored-coded, geometric shapes, along with defined text additions. It is hosted by the National Center for Biotechnology Information (NCBI) at the NCBI-Glycans Page (www.ncbi.nlm.nih.gov/glycans/snfg.html). Several changes have been made to the SNFG page in the past year to update the rules for depicting glycans using the SNFG, to include more examples of use, particularly for non-mammalian organisms, and to provide guidelines for the depiction of ambiguous glycan structures. This Glycoforum article summarizes these recent changes.

The development of glycan-related databases and bioinformatics applications is considerably lagging behind compared with the wealth of available data and software tools in genomics and proteomics. Because the encoding of glycan structures is more complex, most of the bioinformatics approaches cannot be applied to glycan structures. No standard procedures exist where glycan structures found in various species, organs, tissues or cells can be routinely deposited. In this article the concepts of the GLYCOSCIENCES.de portal are described. It is demonstrated how an efficient structure-based cross-linking of various glycan-related data originating from different resources can be accomplished using a single user interface. The structure oriented retrieval options-exact structure, substructure, motif, composition and sugar components-are discussed. The types of available data-references, composition, spatial structures, nuclear magnetic resonance (NMR) shifts (experimental and estimated), theoretically calculated fragments and Protein Database (PDB) entries-are exemplified for Man(3.) The free availability and unrestricted use of glycan-related data is an absolute prerequisite to efficiently share distributed resources. Additionally, there is an urgent need to agree to a generally accepted exchange format as well as to a common software interface. An open access repository for glyco-related experimental data will secure that the loss of primary data will be considerably reduced.

Viruses within a family often vary in their cellular tropism and pathogenicity. In many cases, these variations are due to viruses switching their specificity from one cell surface receptor to another. The structural requirements that underlie such receptor switching are not well understood especially for carbohydrate-binding viruses, as methods capable of structure-specificity studies are only relatively recently being developed for carbohydrates. We have characterized the receptor specificity, structure and infectivity of the human polyomavirus BKPyV, the causative agent of polyomavirus-associated nephropathy, and uncover a molecular switch for binding different carbohydrate receptors. We show that the b-series gangliosides GD3, GD2, GD1b and GT1b all can serve as receptors for BKPyV. The crystal structure of the BKPyV capsid protein VP1 in complex with GD3 reveals contacts with two sialic acid moieties in the receptor, providing a basis for the observed specificity. Comparison with the structure of simian virus 40 (SV40) VP1 bound to ganglioside GM1 identifies the amino acid at position 68 as a determinant of specificity. Mutation of this residue from lysine in BKPyV to serine in SV40 switches the receptor specificity of BKPyV from GD3 to GM1 both in vitro and in cell culture. Our findings highlight the plasticity of viral receptor binding sites and form a template to retarget viruses to different receptors and cell types.

ith more than 53 million confirmed cases of COVID-19 and 1.3 million associated deaths worldwide (World Health Organization, 13 November 2020), there is an urgent need to understand the molecular mechanism of infection and disease to identify patients' susceptibilities and targets for therapeutic intervention. ACE2 is the main viral entry point for coronavirus N63, SARS-CoV and SARS-CoV-2, which cause severe acute respiratory syndromes, the last being responsible for COVID-19 in humans 1-4. ACE2 binds to the S1 domain of trimeric SARS-CoV spike (S) glycoprotein 1 and SARS-CoV-2 S protein 5 , which is primed by TMPRSS2 (ref. 6). Cellular entry of SARS-CoV is dependent on the extracellular domain of ACE2 being cleaved by TMPRSS2 protease at Arg 697 and Lys 716, and the transmembrane domain of ACE2 internalized with the virus 7-9. ACE2 is a carboxypeptidase with several known physiological functions including regulation of blood pressure, salt and water balance in mammals 10,11 , amino acid uptake in the small intestine 12,13 , and glucose homeostasis and pancreatic β-cell function 14,15. Interestingly, ACE2 has been suggested to play an important role in protection from acute lung injury 16-19. ACE2 expression in different tissues is controlled by multiple promoter elements 20-22. In human nasal epithelia and lung tissue, ACE2 expression has been reported to be interferon (IFN) regulated, with evidence of STAT1-, STAT3-, IRF8-and IRF1-binding sites within the ACE2 promoter 23. Activation of IFN-responsive genes is an important antiviral defense pathway in humans, and both interferon and influenza exposure have been reported to increase ACE2 expression in the human airway 23. Bulk and single-cell RNA-sequencing (scRNA-seq) data 24 detect low-level expression of ACE2 in multiple tissues 25. ACE2 expression in the airways is relatively high in nasal epithelium and progressively lower in the bronchial and alveolar regions; this expression profile correlates with levels of infection of SARS-CoV-2 isolates from patients in different airway compartments 26. Consistently, SARS-CoV-2 viral loads have been found to be higher in swabs taken from the nose than swabs taken from the throat of COVID-19 patients 27. Highest ACE2 expression is seen in goblet and ciliated cells of the nasal epithelium 25 , and ACE2 protein localizes to the membrane of motile cilia of respiratory tract epithelia 28. Consistent with this, SARS-CoV-2 has been detected in situ in ciliated airway cells and upper airway epithelium, in addition to pulmonary A novel ACE2 isoform is expressed in human respiratory epithelia and is upregulated in response to interferons and RNA respiratory virus infection