Human papillomavirus type 1 E4 proteins differing by their N-terminal ends have distinct cellular localizations when transiently expressed in vitro

Abstract

Two major human papillomavirus type 1 (HPV-1) E4 proteins are found in large amounts in productively infected differentiating wart cells, a 17-kDa protein translated from an E1-E4 transcript and a processed 16-kDa protein lacking the E1 amino acids at least. The functions of the E4 proteins are still unknown. We have designed an in vitro system allowing the transient expression of three forms of HPV-1 E4 proteins: the 17-kDa E1-E4 protein, an E4 protein without the five E1 amino acids (E4-3200), and E4 protein initiated at the E4 ATG located upstream of the splice acceptor site (E4-3181). The E4-3181 protein has five additional N-terminal amino acids compared with E4-3200. The E4-3181 protein has not yet been detected in vivo but could, in principle, be translated from any transcript encoding the E2 protein. The constructs were transfected in two keratinocyte lines, one derived from a rabbit transplantable carcinoma (VX2R) and the other from a human penile carcinoma (SK-p). E4 transcripts with the expected size were detected in both cell lines by Northern (RNA) blot analysis. Surprisingly, the E4 proteins were found only in the VX2R cells by radioimmunoprecipitation and immunofluorescence experiments. The E1-E4 and the E4-3200 proteins were both cytoplasmic and were associated with granules reminiscent of the cytoplasmic inclusions pathognomonic of the HPV-1 infection. Moreover, each protein showed a specific staining pattern of the inclusions. In contrast, the E4-3181 protein was essentially intranuclear and perinuclear. Thus, HPV-1 E4 proteins differing in their N-terminal ends have distinct cellular localizations and arrangements. It is tempting to assume that this may relate to different roles.