Binding of the human papillomavirus E1 origin-recognition protein is regulated through complex formation with the E2 enhancer-binding protein.

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Abstract
The papillomavirus E1 and E2 proteins form heteromeric complexes and individually bind specific sequences within the viral origin of replication. The mechanism by which these proteins are recruited to the origin and the role of the E1/E2 complex in replication remain undefined. To examine the interplay of these replication proteins, we have analyzed the binding of human papillomavirus (HPV) type 31b E1 and E2 proteins to the origin of replication. Binding of E1 to the origin was increased by E2 proteins and required the presence of E2 binding sites. This increase was due to the formation of E1/E2 complexes which preferentially bound E2-responsive sequences, and the magnitude was determined by the relative affinity of the E2 binding sites. While the E1 protein alone bound an A/T-rich sequence at the HPV-31b origin with low affinity, complexes of E1 and E2 bound instead to E2 binding sites with high affinity. The E1/E2 complex bound a similar sequence as E2 homodimers, but only E2 homodimer binding induced a significant increase in hypersensitivity as indicated by DNase I footprinting. In the presence of excess E1, E1 proteins bound both the A/T-rich sequence and E2 binding sites through complex formation with E2. In E2 excess, E1/E2 complexes preferentially formed, and binding was specific for E2 sites. Therefore, changes in the relative amounts of E1 and E2 proteins can dramatically alter the pattern of binding of viral replication factors to the origin. These observations suggest a model whereby modulation of the relative levels of E1 and E2 during the viral life cycle may alter the pattern of origin binding and possibly episomal copy number.