Merkel cell polyomavirus (MCPyV) is the primary causative agent of Merkel cell carcinoma, a rare but highly aggressive neuroendocrine skin cancer. Large T antigen (LT), one of two oncoproteins encoded by MCPyV, sustains the proliferation of MCPyV-infected tumor cells. LT contains multiple protein-binding motifs that mediate interactions with diverse host proteins essential for its function. Among these, ubiquitin-specific protease 7 (Usp7), a deubiquitinase that regulates the stability of multiple substrates, including p53, is a recently identified LT-interacting protein. In the present study, we characterized the intermolecular interaction between Usp7 and MCPyV LT using biochemical analyses and AlphaFold-based structural modeling. Our results demonstrate that MCPyV LT directly interacts with the TRAF domain of Usp7 via a unique binding motif that is distinct from the canonical sequence. Moreover, MCPyV LT attenuates the p53-deubiquitinating activity of Usp7, providing insights into the molecular function of this viral oncoprotein.

Human papillomaviruses (HPVs) cause abnormal cellular proliferation, leading to malignant or benign lesions, such as cervical cancer and warts. The genome of HPV16, the most prevalent high-risk oncogenic genotype within the Alphapapillomavirus genus, encodes two oncoproteins. One of these proteins, E7, interacts with multiple host proteins and modulates their functions through distinct pathways. The CR2 domain of HPV16 E7 was recently reported to interact with the μ2 subunit of clathrin-adaptor protein 2 (AP2-μ2), an adaptor complex involved in cargo internalization during clathrin-mediated endocytosis. In this study, to provide molecular insights into their intermolecular interactions, we determined the crystal structures of AP2-μ2 in complex with the HPV16 E7-derived peptides. Subsequent biochemical analyses revealed that this interaction is primarily maintained by the Y-x-x-Φ motif and further supported by acidic cluster residues of HPV16 E7. Finally, sequence alignment of the E7 CR2 domains from various HPV genotypes showed that the AP2-μ2-binding motif is largely conserved in Alpha-, Beta-, and Mupapillomaviruses, but not in Nu- and Gammapapillomaviruses.

Lymphocystis disease viruses (LCDVs), members of the Lymphocystivirus genus of the Iridoviridae family, infect various freshwater and marine fish species. They cause the chronic disease lymphocystis, which is non-fatal, but substantially reduces the commercial value of the infected fish. To date, four genotypes of LCDV (LCDV1–4) have been identified, all of which encode the viral homologue of B-cell lymphoma 2 (Bcl-2), a key inhibitor of apoptosis. In this study, we performed biochemical and structural analyses of LCDV2 Bcl-2. Binding assays revealed that LCDV2 Bcl-2 exhibits binding selectivity toward BH3 domain-containing zebrafish proteins. It interacted with zBaxA and zNoxa, but not with zBaxB, zBid, or zBeclin 1, distinguishing it from mammalian and herpesviral Bcl-2 proteins. Subsequent structural determination of LCDV2 Bcl-2 in complex with the BH3 domain of zBaxA demonstrated that they interact in a canonical manner, primarily mediated by the BH3 consensus motif residues of zBaxA. In addition, a subpocket formed by two phenylalanine residues in LCDV2 Bcl-2 plays a key role in determining binding selectivity.