A basic charge cluster near the C‐terminus of the cytoplasmic tail contributes to the molecular stability of human herpesvirus 8 E3 ubiquitin ligases

Abstract

Human herpesvirus 8 (HHV8; also known as Kaposi's sarcoma‐associated herpesvirus [KSHV]) utilizes the viral E3 ubiquitin ligase family members K3 and K5 for immune evasion. Both K3 and K5 mediate the ubiquitination of host MHC class I (MHC‐I) molecules, which play a key role in antigen presentation to cytotoxic T lymphocytes (CTLs). Because ubiquitinated MHC‐I is immediately down‐regulated from the cell surface, HHV8‐infected cells can escape surveillance by CTLs. K3 and K5 have similar domain structures and topologies. They contain an N‐terminal RINGv ubiquitin ligase domain, two transmembrane helices, and an intrinsically disordered cytoplasmic tail at the C‐terminus. The cytoplasmic tail contains a membrane‐proximal “conserved region” involved in ligase activity. On the other hand, the role of the membrane‐distal region of the cytoplasmic tail, termed the “C‐tail” in this study, remains unclear. Here, we demonstrate that the C‐tail contributes to the protein expression of both K3 and K5. The C‐tail‐truncated K3 and K5 mutants were rapidly reduced in cells. The recombinant C‐tail proteins bind to acidic lipids via a basic charge cluster located near the C‐terminus of the C‐tails. Similar to the C‐tail‐truncated mutants, the basic charge cluster‐substituting mutants showed decreased protein expression of K3 and K5. These findings suggest that the basic charge cluster near the C‐terminus of the cytoplasmic tail contributes to the molecular stability of K3 and K5.