INCREASED TRANSFORMING ACTIVITY OF JUNB AND JUND BY INTRODUCTION OF AN HETEROLOGOUS HOMODIMERIZATION DOMAIN

The closely-related proteins c-Jun, JunB and JunD form a family of tra nscription factors which require dimerization for DNA-binding and tran scriptional activity. Dimerization is mediated by a conserved amphipat hic a-helix located adjacent to a highly charged DNA-binding domain. T he Jun proteins can form both homo and heterodimers within the Jun fam ily and can-also cross-dimerize with the Fos proteins. When expressed at high levels in primary chicken cells, each mouse Jun displays disti nct transforming capacities: c-Jun transforms efficiently, JunB transf orms poorly, and JunD does not transform at all, The composition of th e transforming dimers; however, is unknown. To study the activity of J un-Jun homodimers we constructed artificial derivatives, denoted Jun(e b1), the naturally occurring dimerization been replaced by an heterolo gous homodimerization domain from the Epstein-Barr virus transcription factor EB1. These derivatives were introduced into chicken cells and assayed for their ability to affect growth. Unexpectedly, all three Ju n(eb1) proteins conferred a transformed phenotype to primary cultures, promoting sustained growth in low-serum medium and colony formation f rom single cells in agar. These data demonstrate that when forced to a ccumulate as homodimers, both JunB and JunD can transform cells. They also suggest that the poor transforming activity of JunB and the absen ce of transforming activity of JunD may be due to their inability to a ccumulate to high levels as homodimers.