Ubiquitin is a versatile scaffold protein for the generation of molecules with de novo binding and advantageous drug-like properties

• Ubiquitin shows favorable in vivo properties after parenteral application.
• Ubiquitin can bind unspecifically to CXCR4-expressing cell lines.
• Ubiquitin does not elicit signaling via CXCR4-mediated pathways.
• Ubiquitin is not a ligand of CXCR4.
• Ubiquitin is suitable as scaffold for therapeutic applications.

In the search for effective therapeutic strategies, protein-based biologicals are under intense development. While monoclonal antibodies represent the majority of these drugs, other innovative approaches are exploring the use of scaffold proteins for the creation of binding molecules with tailor-made properties. Ubiquitin is especially suited for this strategy due to several key characteristics. Ubiquitin is a natural serum protein, 100% conserved across the mammalian class and possesses high thermal, structural and proteolytic stability. Because of its small size and lack of posttranslational modifications, it can be easily produced in Escherichia coli. In this work we provide evidence that ubiquitin is safe as tested experimentally in vivo. In contrast to previously published results, we show that, in our hands, ubiquitin does not act as a functional ligand of the chemokine receptor CXCR4. Cellular assays based on different signaling pathways of the receptor were conducted with the natural agonist SDF-1 as a benchmark. In none of the assays could a response to ubiquitin treatment be elicited. Furthermore, intravenous application to mice at high concentrations did not induce any detectable effect on cytokine levels or hematological parameters.