Modulation of Lymphotoxin β Surface Expression by Kaposi's Sarcoma-Associated Herpesvirus K3 Through Glycosylation Interference

Kaposi's sarcoma-associated herpesvirus (KSHV) employs diverse mechanisms to subvert host immune responses, contributing to its infection and pathogenicity. As an immune evasion strategy, KSHV encodes the Membrane-Associated RING-CH (MARCH)-family E3 ligases, K3, and K5, which target and remove several immune regulators from the cell surface. In this study, we investigate the impact of K3 and K5 on lymphotoxin receptor (LT beta R) ligands, LT beta and LIGHT, which are type II transmembrane proteins and function as pivotal immune mediators during virus infection. Upon co-expression of viral MARCH proteins with LT beta R ligands, we showed that K3 and K5 selectively targeted LT beta, but not LIGHT, for the downregulation of surface expression. Specifically, K3 and K5 E3 ligases interacted with the transmembrane domain of LT beta. Intriguingly, K3 interacted with an immature form of LT beta, whereas K5 targeted the fully mature form. Subsequent biochemical analyses revealed that K3 disrupted the initial steps of N-glycosylation maturation of LT beta. This interference resulted in the sequestration of LT beta within the endoplasmic reticulum, impeding its trafficking to the plasma membrane. Consequently, the K3-mediated downregulation of LT beta surface expression suppressed the LT beta R downstream signaling pathway. These findings uncover a novel mechanism by which KSHV K3 E3 ligase inhibits the membrane trafficking pathway of the LT beta inflammatory ligand through glycosylation interference, potentially evading LT beta R-mediated antiviral immunity.