Coronavirus Host Cell Attachment and Entry

Coronavirus

Coronaviruses are a group of viruses that contain single-stranded, positive-sense RNA genomes. Several members of this group, which include severe acute respiratory syndrome coronaviruses (SARS-CoV and SARS-CoV-2) and Middle East respiratory syndrome coronavirus (MERS-CoV), are highly pathogenic and have caused significant disease outbreaks in human hosts. In order for human coronaviruses to transcribe and replicate their genomes within host cells, they must first attach and gain entry into host cells using a variety of cell surface receptors and components of the endocytic machinery.

ACE2

ACE2 is a carboxypeptidase that catalyses the conversion of angiotensin I to angiotensin 1-9, or of angiotensin II to the vasodilator angiotensin 1-7 (1). Research studies have identified ACE2 as the receptor for SARS and SARS-CoV-2 coronaviruses (2-4).

DPP4 (CD26)

DPP4 (CD26) is a type II transmembrane glycoprotein expressed ubiquitously in most tissues and different cell types (5,6). In addition to its peptidase activity, DPP4 interacts with multiple important cell surface ligands, such as adenosine deaminase, fibronectin, and IGF2 receptor to influence processes like T cell activation, cell migration, and proliferation (7). Research studies have shown that DPP4 serves as a cellular receptor for the MERS-CoV spike protein (8).

Aminopeptidase N (APN, CD13)

Aminopeptidase N (APN, CD13) is a widely expressed, membrane-bound proteolytic enzyme that breaks down peptides during digestion, cleaves cell surface antigens during antigen presentation, and acts as a receptor for human viruses, including several coronaviruses. This multifunctional protein is implicated in the regulation of many biological processes, including angiogenesis, cell proliferation, cell migration, inflammation, and immune response (9,10).

Basigin (EMMPRIN, CD147)

Basigin (EMMPRIN, CD147) is a type I integral membrane receptor protein belonging to the immunoglobulin superfamily (11). Multiple functions have been ascribed to Basigin; foremost among these is stimulating the secretion of extracellular matrix metalloproteinases by adjacent fibroblasts, a function which has been implicated in promoting tumor progression (12-14). Research studies have suggested that Basigin serves as a novel host cell surface receptor for SARS-CoV-2 (15).

EEA1

EEA1 is an early endosomal marker and a Rab5 effector protein essential for early endosomal membrane fusion and trafficking (16,17). Research studies have shown that efficient coronavirus host cell entry and replication relies upon early endosomes containing EEA1 (18).

Interferon-induced transmembrane protein (IFITM)

Interferon-induced transmembrane protein (IFITM) family members are composed of short amino- and carboxy-termini, two transmembrane domains, and a cytoplasmic domain (19). The primary function of IFITM family proteins appears to be viral restriction, as IFITM proteins inhibit cytosolic entry of coronaviruses by preventing fusion of viral and host membranes (20,21).

Valosin-containing protein (VCP)

Valosin-containing protein (VCP) is a highly conserved and abundant 97 kDa protein that belongs to the AAA family of proteins. These protein complexes participate in many cellular functions, including vesicle transport and fusion, fragmentation and reassembly of the golgi stacks during mitosis, nuclear envelope formation and spindle disassembly following mitosis, cell cycle regulation, DNA damage repair, apoptosis, B and T cell activation, NF-κB-mediated transcriptional regulation, endoplasmic reticulum (ER)-associated degradation, and protein degradation (22). Research studies have shown that VCP facilitates the release of some coronaviruses from the early endosomal compartment (23).

Cathepsin B

Cathepsin B, part of the papain family of proteases, is a widely expressed lysosomal cysteine endopeptidase (24,25). Research studies have suggested that cathepsin B facilitates host cell entry of SARS-CoV by promoting fusion of viral and endosomal membranes (26).

Coronavirus life cycle. This image was adapted and modified from a review by V'kovski et al. 2021. The general lifecycle of coronaviruses can be classified into primarily six steps. 1. The first step is the engagement of the virus to its host receptor. 2. The second step is the entry; SARS-CoV-2 uses either endocytosis or membrane fusion and HCoV-OC43 uses mainly caveolin-mediated endocytosis. 3. The host cell ribosomes in the cytoplasm translate the viralgenome into non-structural proteins (nsps). The nsps serve to build there plication-transcription complex inside double-membrane vesicles (DMVs)derived from the ER. 4. The RNA replication and transcription of subgenomic mRNAs occur simultaneously inside the DMVs. 5. Newly synthesized viral RNA bound to N-protein is condensed and encapsulated within the ERGIG membrane, expressing the viral structural proteins to assemble into mature virions. 6. Virions egress from the host cell through exocytosis. This image wascreated using BioRender.com.

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