Onconase Dimerization Through 3D Domain Swapping: Structural Investigations and Increase Of The Apoptotic Effect In Cancer Cells

Onconase® (ONC), extracted by the oocytes of the frog Rana Pipiens , is a monomeric member of the secretory "pancreatic-type" RNase super-family. Interestingly, ONC is the only monomeric ribonuclease endowed with a high cytotoxic activity. In contrast to other monomeric RNases, ONC displays a high cytotoxic activity. In this work, we found that ONC spontaneously forms dimeric traces and that the dimer amount increases of about four times after lyophilization from acetic acid solutions. Differently from RNase A and the bovine seminal ribonuclease (BS-RNase), which produce N- and C-terminal domain-swapped conformers, ONC forms only one dimer, here named ONC-D. Cross-linking with divinylsulfone reveals that this dimer forms through the three-dimensional domain swapping (3D-DS) of its N-termini, being the C-termini blocked by a disulfide bond. Also, an homology model is proposed for ONC-D, starting from the well-known structure of RNase A N-swapped dimer and taking into account the results obtained from spectroscopic and stability analyses. Finally, we show that ONC is more cytotoxic and exerts a higher apoptotic effect in the dimeric rather than in its monomeric form, either when administered alone or when accompanied by the chemotherapeutic drug gemcitabine. These results suggest new promising implications in cancer treatment.