Proposition of a three-dimensional representation of the constitutive protein of the hepatitis B surface antigen particles.

Hepatitis B surface antigen particles are composed of the major viral envelope protein, the S protein, embedded into a lipid shell. The description of the folding of this protein within the particle membrane could provide helpful information for replacing surface-exposed protein domains by foreign sequences without destabilizing the particle structure. Since the crystallization of the protein in its lipid environment remains inaccessible in the near future, alternative approaches had to be envisaged. We combine here the available experimental structural and topological data with a conformational procedure to identify membrane-associated domains of the HBsAg protein and to propose a three-dimensional description of their assembly within the particle membrane. The proposed protein structure is composed of four membrane-spanning helices and an amphipatic helix located on the inner surface membrane. The transmembrane helices are assembled into a highly hydrophobic complex in which no access to the water environment is allowed. The approach could be extended to other membrane-associated proteins.