The HBV life cycle involves cell entry, viral genome uncoating, replication, transcription, protein expression, reverse transcription, viral particle maturation, and release (Fig.
1). So far, an abundance of studies, using different experimental systems including the cultured cells transfected with plasmid containing a greater-than-unit-length HBV genome, have indicated that several components of the UPS, such as PJA1 [
45] and SMC5/6 [
46], could serve as restriction factors to inhibit the replication cycle of the HBV.
However, during HBV infection, virus-encoded proteins have evolved to interact with host UPS, and could regulate the expression or stability of cellular molecules associated with host UPS to meet the requirement for virus replication in various steps of the life cycle (Fig.
1). Based on hepatoma cells or primary hepatocytes transiently transfected with a plasmid encoding the HBV genome expressing HBx, or the plasmid that contains the mutations preventing HBx expression [
47], HBx was found to play vital roles in different steps of HBV replication. Furthermore, HBx could affect viral replication via a UPS-dependent pathway [
48]. For example, Gao et al. found that HBx could activate and enhance the stability of HBV cccDNA via increasing the expression of MSL2, an E3 ligase that has the capability of reducing the protein level of APOBEC3B [
49]. SMC5/6 has been reported to have the capability of inhibiting HBV transcription [
46]. Furthermore, PJA1, an E3 ligase, binds to SMC5/6 and facilitates the protein complex to eliminate cccDNA to inhibit HBV replication [
45]. As mentioned above, HBx could interact with DDB1 and CUL4 to form a HBx-DDB1-CUL4 E3 ligase complex, and based on the interaction with DDB1 and CUL4, HBx could stimulate viral transcription [
37]. Using substrate-trapping proteomics, Murphy et al. identified that SMC5/6 were the substrates of the HBx-DDB1-CUL4 E3 ligase complex, and HBx degraded SMC5/6 through ubiquitination and the proteasomal pathway to enhance HBV replication [
27]. In addition, the study from Klundert et al. reported Talin-1 (TLN1) was a viral restriction factor that could repress the transcription of the HBV, but HBx was able to relieve its restriction by inducing the degradation of TLN1 mediated by the proteasome pathway to facilitate HBV replication [
50]. In addition, multiple TRIM proteins, which have the function of E3 ligase, including TRIM5, TRIM6, TRIM11, TRIM14, TRIM22, TRIM25, TRIM26, TRIM31, and TRIM41, have been shown to repress HBV transcription [
51,
52]. However, HBx could decrease the expression of TRIM22 at the gene level to facilitate the transcription of viral genes [
53]. In summary, these mentioned studies demonstrate that HBx could promote the replication of HBV via regulating the expression and function of the components of host UPS.
Apart from HBx, HBc has been shown to play a critical role in viral maturation and release [
54], but the precise mechanisms have not been well-assessed. Rost et al. found that HBc could interact with gamma2-Adaptin, an Ub-interacting adaptor, and Nedd4, an E3 ligase, to enhance assembly and particle release of the virus [
55]. However, whether the components of the UPS participate in other steps of the viral life cycle, such as genome uncoating during HBV infection, is still unclear.