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 encoded by highly divergent viruses yet are functionally similar in their ability 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 function, 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 into HPV16 E7 and Ad5 E1A oligomerization properties, sedimentation equilibrium experiments were performed with recombinant HPV16 E7 and Ad5 E1A proteins. These studies reveal that, despite the overall functional similarities between these proteins, monomers, dimers, and tetramers of HPV16 E7 were detected while only reversible monomer-dimer association was identified for Ad5 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 that under physiological protein concentrations HPV16 E7 exists primarily as a dimer. Sedimentation equilibrium experiments of pRB/Ad5 E1A and of pRB/HPV16 E7 complexes demonstrate that the tight association of pRB with the viral 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 to their distinct structures and specific mechanisms of pRB-inactivation.