TI  - Cross-family interaction between the  bHLHZip <prot>USF</prot>  and  bZip <prot>Fra1</prot>  proteins
      results in down-regulation of <prot>AP1</prot> activity.
PG  - 2091-8
AB  - Heterodimerization among the basic-leucine zipper (bZIP) proteins or among
      the basic-helix-loop-helix-leucine zipper (bHLHZip) proteins confers a
      multitude of combinational activities to these transcription factors. To
      further examine the function of the bHLHZip protein, <prot>USF</prot>, we screened for
      cellular proteins which could directly interact with <prot>USF</prot> using the yeast
      two-hybrid system. A bZip protein,  <prot>Fra1</prot> , was found to efficiently interact
      with  <prot>USF</prot> .  <prot>USF</prot>  specifically interacts with  <prot>Fra1</prot>  but not with other closely
      related family members, <prot>c-Fos</prot>, <prot>Fra2</prot>, <prot>FosB</prot>, or with <prot>c-Jun</prot>. Both the bHLHZip
      and the N-terminal regions of  <prot>Fra1</prot>  are required for efficient interaction
      with  <prot>USF</prot> . In vivo association between  <prot>USF</prot>  and  <prot>Fra1</prot>  has been demonstrated
      by co-immunoprecipitation. Expression of exogenous <prot>USF</prot> led to a decrease
      in <prot>AP1</prot>-dependent transcription in F9 cells. Co-expression of exogenous
      <prot>Fra1</prot> restored the <prot>AP1</prot> activity in a dose-dependent manner. These data show
      that  <prot>USF</prot>  and  <prot>Fra1</prot>  physically and functionally interact demonstrating that
      cross-talk occurs between factors of distantly related transcription
      families.
AD  - Centre de Biochimie, Universite de Nice, France.
