bZip Domain 

The bZip DNA binding domain and the closely related helix-loop-helix (HLH) domain are found in many eukaryotic transcription factors. Both bind in the major groove of DNA and have the capability of forming homo- or heterodimers with proteins of the same class. The simplest of these domains is the bZip domain which, when bound to DNA, forms a continuous a-helix. The lower basic part of this helix (the b of bZip) forms sequence specific contacts with DNA whereas the upper part can dimerize with an appropriate partner through the formation of a leucine zipper. The dimer thus has the appearance of a pair of scissors binding to a palindromic DNA sequence in which each half-site is exposed on the opposite face of DNA. The helix-loop-helix domain binds to DNA in a similar manner with the difference that the a-helix involved in dimerization is separated from the DNA binding a-helix by a short loop.