Affinity tag for protein purification and detection based on the disulfide-linked complex of InaD and NorpA

Affinity tags are not only used for the expression and purification of recombinant proteins but also for the detection of protein-protein interactions. Common problems with many affinity tags are excessive length, which may interfere with the structure and function of tagged proteins, and low affinity and/or specificity for primary detection and purification agents. Preliminary results suggest that the C-terminalfive residues of the Drosophila protein NorpA, based on the short, covalent interaction they make with the N-terminal PDZ domain (PDZI) of InaD, are useful as a general affinity tag. First, a PDZI-alkaline phosphatase fusion protein specifically detects both its physiological ligand and a heterologous protein expressing the NorpA C-terminal five residues. The interaction of PDZI with a NorpA-tagged protein is reversible by a reducing agent, which allows nitrocellulose membranes to be stripped completely and reused. In addition, a NorpA-tagged protein can specifically bind to immobilized PDZI resin, while other cellular proteins are washed through. After washing, the NorpA-tagged protein is eluted by a reducing buffer. The NorpA tag's short length makes it the smallest affinity tag available, and its specific and high-affinity interaction with PDZI could yield a powerful system that improves on currently available technology.