Oligomerization properties of the viral oncoproteins adenovirus E1A and human papillomavirus E7 and their complexes with the retinoblastoma protein

Human papillomavirus 16 E7 (HPV16 E7) and adenovirus 5 E1A (Ad5 E1A) are en coded by highly divergent viruses yet are functionally similar in their abi lity to bind the retinoblastoma (pRB) tumor suppressor protein, causing the aberrant displacement of E2F trancription factors. The amino acid residues of HPV16 E7 that are necessary for stability, for inhibition of pRB functi on, and for cell transformation are also necessary for E7 oligomerization. However, neither the specific oligomerization state of HPV16 E7 nor of Ad5 E1A as a function of pRB-binding has been characterized. To gain insight in to HPV16 E7 and Ad5 E1A oligomerization properties, sedimentation equilibri um experiments were performed with recombinant HPV16 E7 and Ad5 E1A protein s. These studies reveal that, despite the overall functional similarities b etween these proteins, monomers, dimers, and tetramers of HPV16 E7 were det ected while only reversible monomer-dimer association was identified for Ad 5 E1A. The apparent Kd(monomer-dimer) of HPV16 E7 is approximately 100-fold lower than that of a comparable region of Ad5 E1A, and it is concluded tha t under physiological protein concentrations HPV16 E7 exists primarily as a dimer. Sedimentation equilibrium experiments of pRB/Ad5 E1A and of pRB/HPV 16 E7 complexes demonstrate that the tight association of pRB with the vira l oncoproteins does not disturb their inherent oligomerization properties. Taken together, this study demonstrates significant differences between the Ad5 E1A and HPV16 E7 oligomerization states that are potentially related t o their distinct structures and specific mechanisms of pRB-inactivation.