The high risk human papillomaviruses (HPVs) are associated with carcinomas of 
cervix and other genital tumors. Previous studies have identified two viral 
oncoproteins E6 and E7, which are expressed in the majority of HPV-associated 
carcinomas. The ability of high risk HPV E6 protein to immortalize human mammary 
epithelial cells has provided a single gene model to study the mechanisms of 
E6-induced oncogenic transformation. In recent years, it has become clear that 
in addition to E6-induced degradation of p53 tumor suppressor protein, other 
targets of E6 are required for mammary epithelial cells immortalization. Using 
the yeast two-hybrid system, we have identified a novel interaction of HPV16 E6 
with protein kinase PKN, a fatty acid- and Rho small G protein-activated 
serine/threonine kinase with a catalytic domain highly homologous to protein 
kinase C. We demonstrate direct binding of high risk HPV E6 proteins to PKN in 
wheat-germ lysate in vitro and in 293T cells in vivo. Importantly, E6 proteins 
of high risk HPVs but not low risk HPVs were able to bind PKN. Furthermore, all 
the immortalization-competent and many immortalization-non-competent E6 mutants 
bind PKN. These data suggest that binding to PKN may be required but not 
sufficient for immortalizing normal mammary epithelial cells. Finally, we show 
that PKN phosphorylates E6, demonstrating for the first time that HPV E6 is a 
phosphoprotein. Our finding suggests a novel link between HPV E6 mediated 
oncogenesis and regulation of a well known phosphorylation cascade.