Three YXXL Sequences of a Bovine Leukemia Virus Transmembrane Protein are Independently Required for Fusion Activity by Controlling Expression on the Cell Membrane

Abstract

Bovine leukemia virus (BLV), which is closely related to human T-cell leukemia viruses, is the causative agent of enzootic bovine leukosis, the most common neoplastic disease of cattle. The transmembrane subunit of the BLV envelope glycoprotein, gp30, contains three completely conserved YXXL sequences that fit an endocytic sorting motif. The two N-terminal YXXL sequences are reportedly critical for viral infection. However, their actual function in the viral life cycle remains undetermined. Here, we identified the novel roles of each YXXL sequence. Syncytia formation ability was upregulated by a single mutation of the tyrosine (Tyr) residue in any of the three YXXL sequences, indicating that each YXXL sequence is independently able to regulate the fusion event. The alteration resulted from significantly high expression of gp51 on the cell surface, thereby decreasing the amount of gp51 in early endosomes and further revealing that the three YXXL sequences are independently required for internalization of the envelope (Env) protein, following transport to the cell surface. Moreover, the 2nd and 3rd YXXL sequences contributed to Env protein incorporation into the virion by functionally distinct mechanisms. Our findings provide new insights regarding the three YXXL sequences toward the BLV viral life cycle and for developing new anti-BLV drugs.

YLMY Tyrosine Residue within the Cytoplasmic Tail of Newcastle Disease Virus Fusion Protein Regulates Its Surface Expression to Modulate Viral Budding and Pathogenicity

“…This difference also exists in bovine leukemia virus (BLV), which possesses three tyrosine motifs. Different motifs and positions are involved in unique regulation functions in syncytium formation, Env packaging, endocytosis, and membrane binding, which suggests this motif regulates viral lifecycles through different mechanisms ( 38 ).…”

PRMT5 Is Required for Bovine Leukemia Virus Infection In Vivo and Regulates BLV Gene Expression, Syncytium Formation, and Glycosylation In Vitro

“…PK15-BLV cells, which were produced by stably transfecting the pig kidney-15 (PK15) cells (National Institutes of Biomedical Innovation, Health and Nutrition: JCRB9040) with CMV∆U3-pBLV-IF2, were cultured in Minimum Essential Medium Eagle (Thermo Fisher Scientific) containing 10% fetal bovine serum and 1% non-essential amino acids (Gibco, Grand Island, NY, USA). CMV∆U3-pBLV-IF2 is the modified version of the BLV-infectious molecular clone pBLV-IF2, which was used previously [ 25 , 49 ]. CMV∆U3-pBLV-IF2 was modified by replacing the U3 region that contains the BLV promoter with the strong CMV promoter to enhance BLV expression.…”

Cytomorphological similarities between feline viral leukemia, bovine enzootic leukosis and adult T-cell leukemia/lymphoma: A review

“…In addition, other diagnostic methods have already been described, such as Western blotting (Cuesta et al, 2018), syncytium induction assay (Sato et al, 2019;Watanuki et al, 2019) and detection of IFA antigens (Matsuura et al, 2019).…”