Hepatitis B virus surface antigen small protein (S-HBsAg) (HBsAg (S protein))

The Hepatitis B virus surface antigen small protein (S-HBsAg) is a glycosylated integral membrane envelope protein that constitutes the viral outer shell and self-assembles with lipids into virions and abundant non-infectious 22-nm subviral particles secreted in great excess over virions.[1][5] HBsAg exists as three size forms encoded by the S open reading frame—large (L), middle (M), and small (S)—that share the C‑terminal S domain comprising four transmembrane helices; the S form lacks preS1/preS2 extensions and is sufficient to form subviral particles used in recombinant vaccines.[1][5] HBsAg mediates multiple steps of the HBV life cycle: it binds the nucleocapsid intracellularly to enable envelopment and egress, helps regulate cccDNA copy number, and contains determinants (notably within preS1 on L-HBsAg) that interact with the hepatocyte receptor NTCP for attachment and entry; antibody-accessible epitopes include the antigenic loop (AGL) targeted by broadly neutralizing antibodies.[1][2][3][5] Structurally, recent cryo-EM of spherical subviral particles shows S-HBsAg forms dimers that assemble into particles with rhombicuboctahedral symmetry and organized protein–lipid architectures, clarifying the arrangement of helices and the AGL on the particle surface.[3][6] Clinically, circulating HBsAg is a key diagnostic and therapeutic biomarker: its presence indicates HBV infection, and loss/seroclearance is considered a functional cure endpoint; excess HBsAg contributes to immune evasion and may drive ER stress and liver pathology.[2][5]

Other names

Hepatitis B surface antigenHBsAgAustralia antigenSmall HBV surface proteinS-HBsAgHBV S protein

Entry inhibition by blocking NTCP–preS1 interaction at the HBV envelope (bulevirtide acts on host NTCP but functionally targets HBsAg-mediated attachment); Neutralization by antibodies binding the antigenic loop (AGL) of HBsAg to prevent virion/particle attachment and entry; Potential vaccine-induced anti-HBs antibodies neutralizing HBsAg-positive particles