The invention provides combinatorial peptide or polypeptide libraries that can become membrane tethered once expressed in cells. The invention additionally provides methods for selecting peptide modulators (e.g., agonists) of GPCRs from the combinatorial libraries of the invention. The invention also provides novel GPCR polypeptide modulators, e.g., biased polypeptide agonists of the glucagon-like peptide 1 receptor (GLP-1R). The invention further provides methods of promoting insulin sensitivity, lowering blood glucose, and reducing body weight as well as methods for treating various diseases such as diabetes and obesity.
