The papillomavirus E1 and E2 proteins are both necessary and sufficient in vivo for efficient origin-dependent viral DNA replication. The ability of E1 and E2 to complex with each other appears to be essential for efficient viral DNA replication. In this study, me used the yeast two-hybrid system and in vitro binding assays to map the domains of the human papillomavirus type 16 (HPV16) E1 and E2 proteins required for complex formation. The amino-terminal 190-amino-acid domain of HPV16 E2 was both required and sufficient for E1 binding. The carboxyl-terminal 229 amino acids of E1 mere essential for binding E2, and the aminoterminal 143 amino acids of HPV16 E1 mere dispensable. Although the ability of the E1 minimal domain (amino acids [aa] 421 to 649) to interact with E2 was strong at 4 degrees C, it was significantly reduced at temperatures above 25 degrees C. A larger domain of E1 from aa 144 to 649 bound E2 efficiently at any temperature, suggesting that aa 144 to 420 of E1 may play a role in the HPV16 E1-E2 interaction at physiological temperatures.