Identification and expression of glycine decarboxylase (p120) as a duck hepatitis B virus pre-S envelope-binding protein

A 120-kilodalton protein (p120) was identified in the duck liver that binds to several truncated versions of duck hepatitis B virus (DHBV) pre-S envel ope protein, suggesting p120 may serve as a DHBV co-receptor. The amino aci d sequences of tryptic peptides from purified p120 were found to be the duc k p protein of the glycine decarboxylase complex (DGD). DGD cDNA cloning re vealed extensive protein conservation with the chicken homologue except for several insertions in the N-terminal leader sequence. The DGD cDNA contain ed no in-frame AUG codon at the predicted initiation site of the open readi ng frame, and site directed mutagenesis experiments established an AUU codo n as the translational initiator. The DGD protein expressed in rabbit retic ulocyte lysates bound truncated DHBV pre-S protein identical to that of p12 0 derived from duck liver confirming DGD as p120. Moreover, transfection st udies in liver- and kidney derived cells revealed both cell surface and cyt oplasmic expression of the protein. Cloning of the glycine decarboxylase cD NA will permit a direct test of whether it functions as a cell surface co-r eceptor or as a co-factor in the DHBV replication cycles.