Hepatitis B Core Protein is Posttranslationally Modified through K29-linked Ubiquitination

Hepatitis B virus (HBV) core protein (HBc) plays many roles in the HBV life cycle such as regulation of transcription, RNA encapsidation, reverse transcription and viral release. To accomplish these functions, HBc interacts with many host proteins and undergoes different posttranslational modifications (PTMs). One of the most common PTM is ubiquitination that was shown to change function, stability, and intracellular localization of different viral proteins, but the role of HBc ubiquitination in the HBV life cycle remains unknown. Here, we found that HBc protein is posttranslationally modified through K29-linked ubiquitination. We performed a series of co-immunoprecipitation experiments with wild type HBc, lysine to arginine HBc mutants and wild type ubiquitin, single lysine to arginine ubiquitin mutants or single ubiquitin-accepting lysine constructs. We observed that HBc protein could be modified by ubiquitination in transfected as well as infected hepatoma cells. In addition, ubiquitination predominantly occurred on HBc lysine 7 and the preferred ubiquitin chain linkage was through ubiquitin-K29. Mass spectrometry (MS) analyses detected UBR5 as a potential E3 ubiquitin ligase involved in K29-linked ubiquitination. These findings emphasize that ubiquitination of HBc may play an important role in HBV life cycle.