Assignment of disulphide bonds in the X protein (HBx) of hepatitis B virus

We have established the disulphide arrangement of cysteines in E. coli expressed HBx by chromatographic analysis of enzymatically cleaved protein and sequence analysis of cysteine containing fragments. Eight of the nine cysteines are disulphide linked in an interesting pattern. Each cysteine is linked to the fourth cysteine in a sequential manner and the last cysteine is free; the disulphide linkages are between Cys7 and Cys78, Cys17 and Cys115, Cys61 and Cys137, Cys69 and Cys143 while Cys148 is free.