From alpha to beta: identification of amino acids required for the N-acetyllactosamine-specific lectin-like activity of bundlin.

Bundle-forming pili (BFP) promote the adherence of typical enteropathogenic Escherichia coli (EPEC) to human intestinal epithelial cells. BFP are polymers of bundlin and nine bundlin alleles have been identified in EPEC isolated from diverse sources. These alleles are divided into two main groups, alpha and beta, based on their amino acid sequences. Alpha bundlins are also N-acetyllactosamine- (LacNAc) specific lectins and bind to HEp-2 cells, whereas beta bundlins do not display these characteristics. The four surface-exposed regions of amino acid sequence heterogeneity between alpha and beta bundlin were therefore investigated as potential LacNAc-specific carbohydrate-binding domains in a bundlin. Mutation of one of these domains, 137-GENNI-141, in alpha(1) bundlin to that of beta bundlin (136-SPDST-140) resulted in BFP that no longer bound to LacNAc or HEp-2 cells. Conversely, mutating the beta3 bundlin gene to encode the alpha bundlin sequence at this domain resulted in the gain of HEp-2 cell adherence. The importance of this domain in carbohydrate binding is supported by the finding that introducing the mutation GENNI-->GENNT altered the alpha1 bundlin carbohydrate-binding specificity from LacNAc to the Lewis X glycan sequence.