Both of the N-Terminal and C-Terminal Regions of Human Papillomavirus Type 16 E7 are Essential for Immortalization of Primary Rat Cells

E7 oncoproteins of mucosal high-risk human papillomavirus type 16 and 18 (HPV16 and HPV18) immortalize primary rodent cells and transform them in collaboration with the activated ras, possibly by interaction with retinoblastoma gene product RB and its related p107. On the other hand, E7 of the cutaneous epidermodysplasia verruciformis-associated HPV5 and HPV8 possess ras-collaborative transformation but not immortalization activity. By using polymerase chain reaction, we constructed chimeric E7 from immortalizing HPV16 E7 and nonimmortalizing HPV5 E7, which have boundaries at the 37/39th, 61/62th, or 79th codon of the HPV16 E7. These chimeric E7 were cloned into the expression vectors to examine their ras-collaboration and immortalization activities. Chimeric E7 that contained N-terminal 39 amino acid residues (R), 61R and 79R of HPV16 E7, showed ras-collaboration activity in primary rat embryo fibroblast and primary baby rat kidney (BRK) cells as efficiently as HPV16 E7. Meanwhile, only the chimeric E7 containing N-terminal 79R of HPV16 E7 was able to immortalize primary BRK cells without second oncogenes. Co-transfection of two chimeric E7 carrying HPV16 N-terminus and HPV16 C-terminus induced immortalization of primary BRK cells. These results suggest that (i) in addition to the N-terminal RB-binding domain, the C-terminal region of HPV16 E7 is essential for immortalization of primary BRK cells, and (ii) two different immortalization functions are present in the two regions of HPV16 E7. By using a yeast two hybrid system, we searched for the HeLa cDNA whose products can bind the C-terminal region of HPV16 E7