Substitutions at position 34 other than Ala also supported binding of some but not all Opa proteins. Y34S substitution was unsuitable for the three Opa proteins of strain C751 but was tolerated by OpaX of MC58. Interestingly, Phe proved to be a more favoured residue in at least two cases with OpaX as well as OpaB binding to Y34F construct at threefold higher levels than the native receptor. Substitutions of Asn at Gln-89 revealed distinct patterns of interactions and binding reduced in the order OpaA > OpaX > OpaB/D (Fig. 3) .