Prevalent conformations and subunit exchange in the biologically active apoptin protein multimer.

Recombinant, bacterially expressed apoptin protein induces apoptosis in human tumour cell lines but not in normal cells, mimicking the behaviour of ectopically expressed apoptin. Recombinant apoptin is isolated exclusively as a highly stable multimeric complex of 30-40 monomers, with little, if any, alpha-helical and beta-sheet structure. Despite its apparent disorder, multimeric apoptin is biologically active. Here, we present evidence that most of the apoptin moieties within the complex may well share a similar conformation. Furthermore, the multimer has extensive and uniform hydrophobic patches and conformationally stable domains. Only a small fraction of apoptin subunits can exchange between multimers under physiologically relevant conditions. These results prompt a model in which the apoptin multimer has a highly stable core of nonexchangeable subunits to which exchangeable subunits are attached through hydrophobic interactions. In combination with previous findings, our results lead us to propose that the stable core of apoptin is the biologically relevant structure.

Apoptin protein multimers form distinct higher-order nucleoprotein complexes with DNA.

The chicken anaemia virus-derived protein apoptin is a tumour-specific cell-killing agent. It is biologically active as a highly stable, multimeric complex, consisting of 30-40 monomers. In tumour cells, but negligibly in normal cells, apoptin is imported into the nucleus prior to the induction of apoptosis. Immunoelectron microscopic data we report here indicate that apoptin predominantly co-localises with heterochromatin and nucleoli within tumour cells. Apoptin's preference for these DNA-dense nuclear bodies may be explained by our finding that apoptin cooperatively forms distinct superstructures with DNA in vitro. These superstructures do not grow beyond a diameter of approximately 200 nm, containing up to 20 multimeric apoptin complexes and approximately 3 kb of DNA. Furthermore, we show a single apoptin multimer to have eight independent, non-specific DNA-binding sites which preferentially bind strand ends, but which can also collaborate to bind longer stretches of DNA. Apoptin's high affinity for naked, undecorated double- and single-stranded DNA and for DNA fibre ends suggests that it may also capture such DNA in superstructures in vivo. Since these forms of DNA are predominantly found in transcriptionally active, replicating and damaged DNA, apoptin could be triggering apoptosis by interfering with DNA transcription and synthesis.

Apoptin induces tumor-specific apoptosis as a globular multimer.

The chicken anemia virus-derived Apoptin protein induces tumor-specific apoptosis. Here, we show that recombinant Apoptin protein spontaneously forms non-covalent globular aggregates comprising 30 to 40 subunits in vitro. This multimerization is robust and virtually irreversible, and the globular aggregates are also stable in cell extracts, suggesting that they remain intact within the cell. Furthermore, studies of Apoptin expressed in living cells confirm that Apoptin indeed exists in large complexes in vivo. We map the structural motifs responsible for multimerization in vitro and aggregation in vivo to the N-terminal half of the protein. Moreover, we show that covalently fixing the Apoptin monomers within the recombinant protein multimer by internal cross-linking does not affect the biological activity of Apoptin, as these fixed aggregates exhibit similar tumor-specific localization and apoptosis-inducing properties as non-cross-linked Apoptin. Taken together, our results imply that recombinant Apoptin protein is a multimer when inducing apoptosis, and we propose that this multimeric state is an essential feature of its ability to do so. Finally, we determine that Apoptin adopts little, if any, regular secondary structure within the aggregates. This surprising result would classify Apoptin as the first protein for which, rather than the formation of a well defined tertiary and quaternary structure, semi-random aggregation is sufficient for activity.