2 NEW P73 SPLICE VARIANTS, GAMMA AND DELTA, WITH DIFFERENT TRANSCRIPTIONAL ACTIVITY

p73 has been recently identified as a new structural and functional ho mologue of the transcription factor p53. It is expressed in either a f ull-length form, alpha, or a shorter beta mRNA variant, with exon 13 s pliced out. Here we report the identification and functional character ization of two new p73 splicing variants, gamma (splicing out exon 11) and delta (splicing out exons 11, 12, and 13). Both gamma and delta p 73 variants are expressed in human peripheral blood lymphocytes, prima ry keratinocytes, and different tumor cell lines, including neuroblast oma, glioblastoma, melanoma, hepatoma, and leukemia. The expression pa ttern of the four p73 splicing variants differs in both primary cells of different lineage and established cell lines even within the same t ype of tumor. A two-hybrid assay was used to characterize the homodime ric and heterodimeric interactions between the p73 variants, and showe d that neither p73 gamma nor p73 delta interact with p53, whereas p73 gamma showed strong interactions with all p73 isoforms, and p73 delta binds efficiently p73 alpha and p73 gamma but only weakly p73 beta. At the functional level, p73 gamma is significantly less efficient in ac tivating transcription of the p21(Waf1/Cip1) promoter than p53 or p73 beta, whereas the effect of p73 delta is intermediate and comparable t o that of p73 alpha. The ability of the different p73 variants to affe ct cell growth in p53 null osteosarcoma SAOS-2 cells correlates with t heir transcriptional activity on the p21(Waf1/Cip1) promoter: p73 beta is the most efficient in inhibiting colony formation, whereas p73 gam ma is almost ineffective. Our results suggest that p73 isoforms may be differentially regulated, with four different isoforms capable of int eracting among themselves and with p53. The relative expression level of each splice variant may modulate p73 transcriptional and growth sup pression activities by affecting heterodimer formation.