STRUCTURE OF PP32, AN ACIDIC NUCLEAR-PROTEIN WHICH INHIBITS ONCOGENE-INDUCED FORMATION OF TRANSFORMED FOCI

pp32 is a nuclear protein found highly expressed in normal tissues in those cells capable of self-renewal and in neoplastic eels. We report the cloning of cDNAs encoding human and murine pp32. The clones encode a 28.6-kDa protein; approximately two-thirds of the N-terminal predic ts an amphipathic alpha helix containing two possible nuclear localiza tion signals and a potential leucine zipper motif. The C-terminal thir d is exceptionally acidic, comprised of approximately 70% aspartic and glutamic acid residues; the predicted pI of human pp32 is 3.81. Human and murine pp32 cDNAs are 88% identical; the predicted proteins are 8 9% identical and 95% similar. Although the structure of pp32 is sugges tive of a transcription factor, pp32 did not significantly modulate tr anscription of a reporter construct when fused to the Ga14 DNA-binding domain. In contrast, in cotransfection experiments, pp32 inhibited th e ability of a broad assortment of oncogene pairs to transform rat emb ryo fibroblasts, including ras + myc, ras + jun, ras + E1a, ras + muta nt p53, and E6 + E7. In related experiments, pp32 inhibited the abilit y of Rat 1a-myc cells to grow in soft agar, whereas it failed to affec t ras-induced focus formation in NIH3T3 cells. These results suggest t hat pp32 may play a key role in self-renewing cell populations where i t may act in the nucleus to limit their sensitivity to transformation.