Engineering a Minimal Leucine-rich Repeat IgG-binding Module.

Sea lamprey immunization can yield leucine-rich repeat (LRR) protein binders analogous to globular antibodies developed from mammals. A novel minimal LRR was discovered through lamprey immunization with human immunoglobulin G Fc domain (IgG Fc). Initial attempts to solubly express this LRR protein, VLRB.IgGFc, in Escherichia coli proved challenging, so it was analyzed using the cell-free method ribosome display. In ribosome display, VLRB.IgGFc was found to bind specifically to the Fc domain of IgG, with little observed cross-reactivity to IgA or IgM. The minimal repeat protein architecture of VLRB.IgGFc may facilitate modular LRR extensions to incorporate additional or augmented functionality within a continuous, structurally defined scaffold. We exploited this modularity to design a chimera of a well-characterized, soluble LRR repebody and the initially insoluble VLRB.IgGFc to produce soluble Repe-VLRB.IgGFc. The minimal IgG Fc-binding module, Repe-VLRB.IgGFc, and future-engineered variants thereof should be useful additions to the biotechnological toolbox for detecting, purifying, or targeting IgGs. More generally, this two-step approach of minimal LRR binder discovery via sea lamprey immunization followed by modular augmentation of functionality may be of general utility in protein engineering.