Murine leukemia virus glycosylated Gag reduces murine SERINC5 protein expression at steady-state levels via endosome/lysosome pathway to counteract the SERINC5 antiretroviral activity.

Glycosylated Gag (glycoGag) is an accessory protein expressed by most gamma-retroviruses including murine leukemia virus (MLV). MLV glycoGag not only enhances MLV replication and disease progression but also increases human immunodeficiency virus type 1 (HIV-1) infectivity as Nef does. Recently, SERINC5 (Ser5) was identified as the target for Nef, and the glycoGag Nef-like activity has been attributed to the Ser5 antagonism. Here, we investigated how gly-coGag antagonizes Ser5 using MLV glycoMA and murine Ser5 proteins. We confirm previous observations that glycoMA re-localizes Ser5 from plasma membrane to perinuclear punctated compartments and the important role of its Y36XXL39 motif in this process. We find that gly-coMA decreases Ser5 expression at steady-state levels, and identify two other glycoGag crucial residues P31 and R63 for the Ser5 downregulation. The glycoMA and Ser5 interaction is detected in live cells using a bimolecular fluorescence complementation (BiFC) assay. Ser5 is internalized via receptor-mediated endocytosis, and re-localized to Rab5+ early, Rab7+ late, and Rab11+ recycling endosomes by glycoMA. Although glycoMA is not polyubiquitinated, the Ser5 down-regulation requires Ser5 polyubiquitination via the K48- and K63-linkage, resulting in Ser5 destruction in lysosomes. Although P31, Y36, L39, and R63 are not required for glycoMA interaction with Ser5, they are required for Ser5 re-localization to lysosomes for destruction. In addition, although murine Ser1, Ser2, and Ser3 exhibit very poor antiviral activity, they are also targeted by glycoMA for lysosomal destruction. We conclude that glycoGag has a broad activity to downregulate SERINC proteins via the cellular endosome/lysosome pathway, which promotes viral replication. [ABSTRACT FROM AUTHOR]