MPP5 (PALS1), a tight junction-associated protein, is a member of the CRUMBS3:MPP5:PATJ polarity complex, which is crucial for the establishment and maintenance of epithelial polarity in mammals. MPP5 has multiple protein-protein interaction domains including a PSD-95/Dlg/ZO-1 (PDZ) domain. A functional PDZ domain-binding motif has been identified at the carboxy-terminus end of SARS-CoV-1 E (Teoh KT et al. 2010; Jimenez-Guardeño JM et al. 2014) and SARS-CoV-2 E (Caillet-Saguy C et al. 2021; Chai J et al. 2021; Shepley-McTaggart A et al. 2021). SARS-CoV-2 E and SARS-CoV-1 E are abundantly expressed inside the infected cell and actively involved in the pathogenic viral mechanisms (Venkatagopalan P et al. 2015; Wong NA & Saier MH Jr 2021). The viral E localizes to the endoplasmic reticulum-Golgi intermediate compartment (ERGIC) for viral assembly and budding. NMR spectroscopy identified the pentameric structure of transmembrane domain of SARS‐CoV‐2 E protein in lipid bilayers (Mandala VS et al. 2020). Infection with recombinant SARS-CoV-1 lacking an E protein PDZ domain-binding motif was attenuated in mice, which was accompanied by a decreased expression of inflammatory cytokines during infection, and a substantial increase of survival (Jimenez-Guardeño JM et al. 2014). SARS-CoV-1 E was found to interfere with host PDZ‐dependent interactions of MPP5 affecting cellular mechanisms thus favoring viral replication (Teoh KT et al. 2010). Coimmunoprecipitation and pull-down assays showed that the viral E protein interacted with MPP5 in human embryonic kidney 293T (HEK293T) and African green monkey kidney Vero E6 cells (Teoh KT et al. 2010). The last four carboxy-terminal amino acids (DLLV) of E interacted with the PDZ domain of MPP5 (Teoh KT et al. 2010). Immunofluorescence assay showed that MPP5 was mainly localized at cell-cell junctions in monolayers of non infected Vero E6 cells, while the binding of SARS-CoV-1 E to MPP5 (PALS1) redistributed MPP5 from the tight junctions of the lung epithelium to the ER-Golgi intermediate compartment (ERGIC) where E assembles in SARS-CoV-1-infected Vero E6 cells (Teoh KT et al. 2010). Ectopic expression of E in madin-darby canine kidney II (MDCKII) epithelial cells significantly altered cyst morphogenesis and, furthermore, delayed formation of tight junctions, affected polarity, and modifies the subcellular distribution of MPP5, in a PDZ-binding motif-dependent manner (Teoh KT et al. 2010). Altogether, these findings suggest that SARS-CoV-1 E interacts with MPP5 (PALS1) and most likely retains MPP5 at the virus assembly site in mammalian cells. However, the relevance of this interaction during virus infection and its impact on virulence in vivo was not tested.

A sequence comparison of the E proteins of the pathogenic human CoVs, SARS-CoV-1, MERS-CoV, and SARS-CoV-2, demonstrated a very high sequence similarity between SARS-CoV-1 E and SARS-CoV-2 E (Grifoni A et al. 2020; Schoeman D & Fielding BC 2020; Hassan SS et al. 2020). This similarity is not shared with the MERS-CoV E protein (Schoeman D & Fielding BC 2020). In silico structural modelling and analyses of E proteins suggested a strengthened binding of SARS-CoV-2 E protein with MPP5 (De Maio F et al. 2020). Equilibrium and kinetic binding experiments of peptides mimicking the E protein of SARS-CoV-1 and SARS-CoV-2 to the PDZ domain of MPP5 further support the hypothesis that the increased virulence of SARS-CoV-2 compared to SARS-CoV-1 may rely on the increased affinity of its E protein for MPP5 (Toto A et al. 2020). However, these binding affinity measurements (Toto A et al. 2020) differ from the findings that the C-terminal PDZ binding motifs of SARS-CoV-1 and SARS-CoV-2 E proteins bind the MPP5 PDZ domain with comparable affinities (Javorsky A et al. 2021). Further, structural studies of the SARS-CoV-2 E:MPP5 complex revealed that the C-terminal DLLV motif of E recognizes a pocket formed by hydrophobic residues from the PDZ and SH3 domains of MPP5 (PALS1) (Chai J et al. 2021; Javorsky A et al. 2021). Similar structural data were reported for the SARS-CoV-1 E protein (Javorsky A et al. 2021).

This Reactome event shows interaction between SARS-CoV-2 E pentamer and human MPP5.