Four disulphide bonds in PkNBPXb are important for expression on the surface of COS cells. The N-terminal domain of PkNBPXb contains five cysteine residues (C193, C254, C298, C326, C332). Using site directed mutagenesis, all five residues were individually mutated to glycine and all but C193 were found to be required for binding to erythrocytes. The glycine residues present in each construct were mutated back to cysteine residues; these constructs were able to express protein on the surface and also able to bind erythrocytes. Surface and internal expression is indicated with + or – signs.