Nanobodies restore stability to cancer-associated mutants of tumor suppressor protein p16INK4a

We describe the generation and characterization of camelid single-domain antibodies (nanobodies) raised against tumor suppressor protein p16INK4a (p16). p16 is a cell cycle regulator that inhibits cyclin-dependent kinases CDK4 and CDK6 and is inactivated in sporadic and familial cancers. The majority of p16 missense mutations cause loss of function by destabilizing the protein's structure. We identify nanobodies that bind p16 with nanomolar affinities and restore the stability of many different cancer-associated p16 mutations located at sites throughout the protein. The crystal structure of a nanobody-p16 complex reveals that the nanobody binds to the opposite face of p16 to the CDK-binding interface permitting formation of a ternary complex. We confirm that nanobodies bind to p16 in a cellular setting and do not preclude p16 binding to CDK6 and its ability to induce cell-cycle arrest. These findings indicate that nanobodies merit testing as pharmacological chaperones for p16 reactivation in the cell.