Fabs offer an attractive platform for monoclonal antibody discovery/engineering, but library construction can be cumbersome. We report a simple method - Golden Gate assembly with a bi-directional promoter (GBid) - for constructing phage display Fab libraries. In GBid, the constant domains of the Fabs are located in the backbone of the phagemid vector and the library insert comprises only the variable regions of the antibodies and a central bi-directional promoter. This vector design reduces the process of Fab library construction to "scFv-like" simplicity and the double promoter ensures robust expression of both constituent chains. To maximize the library size, the 3 fragments comprising the insert - two variable chains and one bi-directional promoter - are assembled via a 3-fragment overlap extension PCR and the insert is incorporated into the vector via a high-efficiency one-fragment, one-pot Golden Gate assembly. The reaction setup requires minimal preparatory work and enzyme quantities, making GBid highly scalable. Using GBid, we constructed a chimeric chicken-human Fab phage display library comprising 1010 variants targeting the multi-transmembrane protein human CD20 (hCD20). Selection/counter-selection on transfected whole cells yielded hCD20-specific antibodies in four rounds of panning. The simplicity and scalability of GBid makes it a powerful tool for the discovery/engineering of Fabs and IgGs.
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