The human papillomavirus type 11 E1^E4 protein is a transglutaminase 3 substrate and induces abnormalities of the cornified cell envelope

The human papillomavirus (HPV) E1^E4 protein is detected in the cytoplasm of differentiated keratinocytes, near the cornified cell envelope. HPV does not induce lysis of the infected keratinocyte, and the normally durable cornified cell envelope that forms during keratinocyte differentiation would seemingly inhibit viral egress. HPV infection induces abnormalities of the cornified cell envelope, but the exact mechanisms involved are not well understood. We tested whether the HPV 11 E1^E4 protein, which co-localizes the cell envelope and co-purifies with cell envelope fragments, could serve as an in vitro substrate for transglutaminases. We found evidence of E1^E4 cross-linking by endogenous transglutaminases in an in situ assay using frozen sections of human foreskin, and in addition, E1^E4 protein was cross-linked by recombinant transglutaminase 3 (but not transglutaminase 1) in an in vitro cross-linking assay. We also tested whether expression of E1^E4 in differentiated keratinocytes would induce morphologic alterations of cornified cell envelopes. Differentiated keratinocytes expressing E1^E4 were disorganized and pleomorphic compared to control cells, and cell envelopes purified from E1^E4-expressing cells were small, fragmented, and rough bordered compared to the round, smooth bordered cell envelopes from control cells. We conclude from these in vitro experiments that the E1^E4 protein is cross-linked by transglutaminase 3, and that E1^E4 expression in differentiated keratinocytes induces morphologic abnormalities of the cornified cell envelope.