The effect of the HPV-16 minor capsid protein L2 on the HPV-16 viral transcription regulator E2

The nucleus contains a variety of morphologically distinct substructures called nuclear bodies, which include the promyelocytic leukemia oncogenic domains (PODs) also known as PML-NDIO. PODs are macromolecular multiprotein complexes that are present in all cultured cell lines as well as in vivo. The major component of PODs is the PML protein, which was originally identified as the fusion partner of retinoic acid receptor alpha (RARa) in the chromosomal translocation t(15;17) in patients with acute promyelocytic leukaemia (APL) (Kakizuka et al, 1991; Lavau et al, 1991; Goddard et al, 1992). The minor capsid proteins L2 of BPV-1, HPV-11 and HPV-33 have been shown to localise to PODs in the absence of other viral components (Day et al., 1998) and coexpression of BPV-1 12 with BPV-1 E2TA recruits E2 to PODs (Lambert et al, 2000). The presence of L2 in PODs also appears to be associated with the recruitment of the major capsid protein LI, the association of PODs with E2 is dependent on L2 but is independent of LI. The effect of HPV-16 L2 on the functions of HPV-16 E2 and the implications of this interaction to the virus life cycle are discussed. This study showed that HPV-16 L2 has a selective effect on the functions of HPV16 E2. L2 was able to down regulate the transcription transactivation function of E2 in HaCaT, U20S and C33a cells. No effect of L2 on E2 mediated DNA replication was observed. L2 was also able to reduce the level of E2 expression in HaCaT and U20S cells but not in C33a cells. The effect of L2 on E2 expression in HaCaT cells was further investigated by examining E2 mRNA levels and protein half-life. No difference in E2 mRNA or protein half-life was detected in the presence of L2. A series of L2 amino and carboxyl terminal deletion mutants were constructed as GST fusion proteins and GST binding assays were performed which showed that the amino terminus ofL2, even just the first 50 amino acids, was capable of binding with E2. GFP fusion forms of each L2 deletion mutant were also constructed and cellular localisation detected by immunofluorescence. GFP-L2 and all C terminal deletion mutants localised and were retained in the nucleus while the N-terminal deletion mutants localised to both the nucleus and the cytoplasm. Investigation of the effect of L2 deletion mutants on the transcription transactivation function of E2 showed that mutants expressing 1-200 and 150-473 amino acids of L2 do not down regulate function in HaCaT and C33a cells. In HaCaT cells, mutants expressing amino acids 1-50 and 1-100 also did not inhibit E2 function indicating that binding to E2 did not correlate with down regulation of transcription transactivation. Furthermore, only full-length L2 was able to reduce the level of E2 expression