In vivo biotinylated proteins as targets for phage-display selection experiments

Screening phage-displayed combinatorial libraries represents an attractive method for identifying affinity reagents to target proteins. Two critical components of a successful selection experiment are having a pure target protein and its immobilization in a native conformation. To achieve both of these requirements in a single step, we have devised cytoplasmic expression vectors for expression of proteins that are tagged at the amino- or carboxy-terminus (pMCSG16 and 15) via the AviTag, which is biotinylated in vivo with concurrent expression of the BirA biotin ligase. To facilitate implementation in high-throughput applications, the engineered vectors, pMCSG15 and pMCSG16, also contain a ligase-independent cloning site (LIC), which permits up to 100% cloning efficiency. The expressed protein can be purified from bacterial cell lysates with immobilized metal affinity chromatography or streptavidin-coated magnetic beads, and the beads used directly to select phage from combinatorial libraries. From selections using the N-terminally biotinylated version of one target protein, a peptide ligand (Kd= 9 microM) was recovered that bound in a format-dependent manner. To demonstrate the utility of pMCSG16, a set of 192 open reading frames were cloned, and protein was expressed and immobilized for use in high-throughput selections of phage-display libraries.