The Fos and Jun families of eukaryotic transcription factors heterodimerize to
form complexes capable of binding 5'-TGAGTCA-3' DNA elements. We have determined
the X-ray crystal structure of a heterodimer of the bZIP regions of c-Fos and
c-Jun bound to DNA. Both subunits form continuous alpha-helices. The
carboxy-terminal regions form an asymmetric coiled-coil, and the amino-terminal
regions make base-specific contacts with DNA in the major groove. Comparison of
the two crystallographically distinct protein-DNA complexes show that the
coiled-coil is flexibly joined to the basic regions and that the Fos-Jun
heterodimer does not recognize the asymmetric 5'-TGAGTCA-3' recognition element
in a unique orientation. There is an extensive network of electrostatic
interactions between subunits within the coiled-coil, consistent with proposals
that these interactions determine preferential formation of the heterodimer over
either of the homodimers.
