Receptor-type tyrosine-protein phosphatase C, also known as CD45, is a tyrosine-protein phosphatase required for T-cell activation through the antigen receptor []. It acts as a positive regulator of T-cell co-activation upon binding to DPP4 (CD26) [].
The CoV Spike (S) protein is an envelope glycoprotein that plays the most important role in viral attachment, fusion, and entry into host cells, and serves as a major target for the development of neutralizing antibodies, inhibitors of viral entry, and vaccines. It is synthesised as a precursor protein that is cleaved into an N-terminal S1 subunit (~700 amino acids) and a C-terminal S2 subunit (~600 amino acids) that mediates attachment and membrane fusion, respectively. Three S1/S2 heterodimers assemble to form a trimer spike protruding from the viral envelope. The S1 subunit contains a receptor-binding domain (RBD), while the S2 subunit contains a hydrophobic fusion peptide and two heptad repeat regions. S1 contains two structurally independent domains, the N-terminal domain (NTD) and the C-terminal domain (C-domain). Depending on the virus, either the NTD or the C-domain can serve as the receptor-binding domain (RBD). Most CoVs, including SARS-CoV-2, SARS-CoV, and MERS-CoV use the C-domain to bind their receptors. However, CoV such as mouse hepatitis virus (MHV) uses the NTD to bind its receptor, mouse carcinoembryonic antigen related cell adhesion molecule 1a (mCEACAM1a). The S1 NTD contributes to the Spike trimer interface [, , , , ].This entry represents the receptor-binding domain (RBD) of the S1 subunit of the spike (S) protein from Tylonycteris bat coronavirus HKU4 and other Middle East Respiratory Syndrome (MERS)-related coronaviruses which are phylogenetically closely related, sharing high sequence similarity. HKU4 is able to bind the MERS-CoV receptor, human dipeptidyl peptidase 4 (DPP4), also called CD26 but with lower affinity than MERS-CoV which indicates that HKU4 is less adapted to human DPP4 [].
