Expression of the cyclin dependent kinase inhibitor p27 KIP1 is intimately linked to the control of proliferation, and is itself regulated by transcription, translation, phosphorylation, protein stability or sequestration. p27 KIP1 is also regulated during apoptosis; cleavage occurs at DPSD 139 S and ESQD 108 V, by a sub-set of Z-VAD-fmk-sensitive caspases. We have identi®ed a related but distinct mechanism that regulates p27 KIP1 in proliferating lymphoid cell lines. In a B-lymphoid cell line (BJAB), the abundance of p27 KIP1 oscillates inversely to proliferation; loss of full-length p27 KIP1 correlates with the appearance of a truncated version corresponding to cleavage at DPSD

The viral bZIP transcription factor Zta (BZLF1, EB1, ZEBRA) mediates the switch between the latent and lytic cycles of Epstein-Barr virus (EBV). In part, its activity requires the formation of homodimers and interaction with specific DNA sequence elements (ZREs). Zta has an atypical zipper motif that has a lower stability than do typical bZIP proteins. Here we show that a synthetic peptide directed against the zipper can disrupt the DNA-binding function of Zta. This highlights the relevance of this region for the function of Zta and demonstrates that the zipper region is a potential target for therapeutic agents. We also unmask the relevance of a region adjacent to the zipper (CT region), which is required to direct the interaction of Zta with DNA and to transactivate ZRE-dependent promoters in vivo.

The cyclin-dependent kinase inhibitor p27 KIP1 plays a key role in controlling cell proliferation. Here we show that p27 KIP1 is commonly down-regulated in B-cells immortalized by Epstein-Barr virus (EBV) (lymphoblastoid cell lines, LCLs). The significance of this event for the immortal phenotype of LCLs is implied by a requirement for active cdk2-containing complexes for continued proliferation, and by the ability of the residual p27 KIP1 to associate with cdk2. The mechanism of p27 KIP1 attenuation is post-translational, but inhibitor studies reveal that the mechanism does not rely heavily on the proteasome. Instead we find that LCLs contain an activity that cleaves a caspase recognition site present in p27 KIP1 (DPSD 139 ). The activity is not associated with apoptosis and closely resembles a proliferation-associated caspase activity we previously described in the EBVnegative B-lymphoma-derived cell line BJAB. Importantly, proliferating LCLs contain a p27 KIP1 product that is consistent with cleavage at this site. Inhibition of caspase(s) in vivo modulates p27 KIP1 expression and strongly inhibits proliferation of IB4 cells. This inhibitor profile is identical to that displayed by the DPSD-directed caspase present in BJAB cells, suggesting that the caspase may fulfil a general role in controlling p27 KIP1 expression in immortal lymphoid cell lines. Thus, apoptosis-independent cleavage appears to contribute to the maintenance of the low basal levels of p27 KIP1 in B-cells immortalized by EBV.