BIP BINDING SEQUENCES IN ANTIBODIES

During the process of folding and assembly of antibody molecules in th e endoplasmic reticulum, immunoglobulin heavy and light chains associa te transiently with BiP, a resident endoplasmic reticulum protein that is a member of the Hsp70 family of molecular chaperones. Dip is thoug ht to recognize unfolded or unassembled polypeptides by binding extend ed sequences of approximately seven amino acids that include bulky hyd rophobic residues not normally exposed on the surface of native protei ns. We used a computer algorithm developed to predict BiP binding site s within protein primary sequences to identify sites within immunoglob ulin chains that might mediate their association with BiP. Very few of the sequential heptapeptides in the heavy or light chain sequences we re potential Dip binding sites. Analysis of the ability of synthetic h eptapeptides corresponding to 24 potential sites in heavy chains to st imulate the ATPase activity of BiP indicated that at least half of the m were authentic Dip binding sequences. These sequences were not confi ned to a single domain of the heavy chain but were distributed within both the V-H and C-H domains. interestingly, when the BiP binding sequ ences were mapped onto the three dimensional structure of the Fd antib ody fragment, the majority involve residues that participate in contac t sites between the heavy and light chains. Therefore, we suggest that in. vivo BiP chaperones the folding and assembly of antibody molecule s by binding to hydrophobic surface regions on the isolated immunoglob ulin chains that subsequently participate in interchain contacts.