The genital tract human papillomaviruses (HPVs) can be separated into two groups based on their clinical associations and risk for malignant progression. The high risk HPVs (such as HPV-16 and HPV-18) that are associated with specific anogenital cancers encode two oncoproteins E6 and E7, which are expressed in the HPV positive cancers. The E7 protein functions in cellular transformation, at least in part, through interactions with the product of the retinoblastoma susceptibility gene, pRB, and the other pRB related """"""""pocket proteins"""""""". The major target of the E6 oncoprotein encoded by the genital tract, cancer associated human papillomaviruses is the p53 tumor suppressor protein. Several lines of evidence suggest that E6 has targets important in its oncogenic phenotype in addition to p53. The binding of E6 to p53 is mediated by a cellular protein that we have previously identified and characterized, called the E6 associated protein (E6AP). In the presence of B6, E6AP catalyses the ubiquitination and proteolysis of p53. E6AP is an E3 ubiquitin protein ligase that is expressed in all tissues and functions as an E3 enzyme in the absence of E6. We have preliminary data identifying cellular proteins that are E independent substrates of E6AP. These include HHR23A, one of the human homologues of the yeast RAD23 protein, and several members of the Src tyrosine kinase family. We will explore the possibility that HPV E6 may affect the regulation of the stability of the normal cellular substrates of E6AP. The altered stability of these substrates could be important to the transformation functions of this viral oncoprotein. B6AP has also recently been identified as the candidate gene for Angelman Syndrome, a human genetic neurologic disorder characterized by mental retardation, seizures, and a gait disorder. It is hypothesized that defects in the ubiquitin-protein ligase activity of E6AP may be involved in its pathogenesis, perhaps through the stabilization of proteins that normally serve as its ubiquitination substrates. The identification of the cellular substrates of E6AP may provide insight into our understanding of the pathogenesis of AS. The candidate E6AP substrates that we hake identified will be verified by examining their respective stabilities in mouse cells in which the E6AP gene has been knocked out.
