Formation of a disulfide bond in the immunoglobulin domain of the myelin P0 protein is essential for its adhesion.

It is widely accepted, although never demonstrated, that the formation of a disulfide bond in the majority of immunoglobulin (Ig)-like domains stabilizes their final conformation and thus is essential to their functioning as adhesion/recognition molecules. The myelin P0 protein, which has been shown directly to behave as a homophilic adhesion molecule, contains a single Ig-like domain, stabilized by a putative Cys21-Cys98 disulfide bond. To test if this bond is indeed necessary to the adhesive function of P0, the nucleotides in the P0 cDNA coding for Cys21 were altered to code for an alanine. The mutated P0 cDNA was transfected into Chinese hamster ovary cells, expression of the mutated P0 protein was characterized, and the adhesiveness of Cys21-mutated P0-expressing cells and that of cells expressing equivalent surface amounts of the unmutated protein were compared. It was found, as we previously reported, that incubation of a single cell suspension of the unmutated P0-expressing cells resulted in the rapid formation of large aggregates. In contrast, after a similar incubation the cells expressing the Cys21-mutated P0 were still mostly single cells, a result indistinguishable from that observed with the control transfected cells. This suggests that the P0 protein, when mutated at Cys21, does not behave as a homophilic adhesion molecule, which in turn implies that the formation of an Ig domain disulfide bond is essential to the functioning of this molecule.