Protein domain mapping by λ phage display: The minimal lactose-binding domain of galectin-3

Mapping of protein domains having a distinct function is essential to understanding the protein's structure-function relationship. We used a bacteriophage λ surface expression vector, λfoo, in order to determine the minimal carbohydrate-binding domain of human galectin-3 (Gal-3). Gal-3 cDNA was randomly digested by DNase I and cloned into the phage vector. The library generated was screened by affinity selection using lactose immobilized on agarose beads. DNA sequence analysis of a set of isolated clones defined the minimal folding domain of Gal-3 required for lactose binding, which consisted of 136 amino-acid residues. Using the phage clones isolated, we also determined relative dissociation constants in solution between lactose and the minimal domain expressed on the phage surface. This technique does not require either purified or labeled proteins, and bacteriophage λ surface display may, therefore, be useful for protein domain mapping and in vitro studies of various macromolecular interactions.