Overall, the data suggest that Opa binding to the receptor requires an extended aliphatic chain at position 91 and certain arrangements of the chain may facilitate binding of most Opa proteins (Ile = leu > Thr). At position Α,Β D, E β−strand: (Virji et al., 1999) . Side chains of the residues introduced at positions 34, 89 and 91 in this study are shown in B for comparison 34, the removal of the hydroxyl residue (Y43F) is tolerated or preferred whereas the absence of aromatic ring (Y34S) has an overall deleterious effect on Opa binding. Finally, reduction of the side chain extension (Q89N) reduces binding of three of the four Opa proteins.