A soluble form of the avian hepatitis B virus receptor - Biochemical characterization and functional analysis of the receptor ligand complex

Avian hepatitis B virus infection is initiated by the specific interaction of the extracellular preS part of the large viral envelope protein with car boxypeptidase D (gp180), the primary cellular receptor. To functionally and biochemically characterize this interaction, we purified a soluble form of duck carboxypeptidase D from a baculovirus expression system, confirmed it s receptor function, and investigated the contribution of different preS se quence elements to receptor binding by surface plasmon resonance analysis. We found that preS binds duck carboxypeptidase D with a 1:1 stoichiometry, thereby inducing conformational changes but not oligomerization, The associ ation constant of the complex was determined to be 2.2 x 10(7) M-1 at 37 de grees C, pH 7.4, with an association rate of 4.0 x 10(4) M-1 s(-1) and a di ssociation rate of 1.9 x 10(-3) s(-1), substantiating high affinity interac tion of avihepadnaviruses with their receptor carboxypeptidase D, The separ ately expressed receptor-binding domain, comprising about 50% of preS as de fined by mutational analysis, exhibits similar constants. The domain consis ts of an essential element, probably responsible for the initial receptor c ontact and a part that contributes to complex stabilization in a conformati on sensitive manner. Together with previous results from cell biological st udies these data provide new insights into the initial step of hepadnaviral infection.