The SEP domain of p47 acts as a reversible competitive inhibitor of cathepsin L.

The solution structure of the human p47 SEP domain in a construct comprising residues G1-S2-p47(171-270) was determined by NMR spectroscopy. A structure-derived hypothesis about the domains' function was formulated and pursued in binding experiments with cysteine proteases. The SEP domain was found to be a reversible competitive inhibitor of cathepsin L with a Ki of 1.5 microM. The binding of G1-S2-p47(171-270) to cathepsin L was mapped by biochemical assays and the binding interface was investigated by NMR chemical shift perturbation experiments.