Apoptin, a protein derived from the chicken anaemia virus, induces cell death in 
various cancer cells but shows little or no cytotoxicity in normal cells. The 
mechanism of apoptin-induced cell death is currently unknown but it appears to 
induce apoptosis independent of p53 status. Here we show that p73, a p53 family 
member, is important in apoptin-induced apoptosis. In p53 deficient and/or 
mutated cells, apoptin induced the expression of TAp73 leading to the induction 
of apoptosis. Knockdown of p73 using siRNA resulted in a significant reduction 
in apoptin-induced cytotoxicity. The p53 and p73 pro-apoptotic target PUMA plays 
an important role in apoptin-induced cell death as knockdown of PUMA 
significantly reduced cell sensitivity to apoptin. Importantly, apoptin 
expression resulted in a marked increase in TAp73 protein stability. 
Investigation into the mechanisms of TAp73 stability showed that apoptin induced 
the expression of the ring finger domain ubiquitin ligase PIR2 which is involved 
in the degradation of the anti-apoptotic ∆Np73 isoform. Collectively, our 
results suggest a novel mechanism of apoptin-induced apoptosis through increased 
TAp73 stability and induction of PIR2 resulting in the degradation of ∆Np73 and 
activation of pro-apoptotic targets such as PUMA causing cancer cell death.