Homeodomain–Leucine Zipper Transcription Factors: Structural Features of These Proteins, Unique to Plants

The homeodomain is encoded by a 180 bp consensus DNA sequence named the homeobox, and it is present in transcription factors (TFs) involved in developmental processes in eukaryotic kingdoms. One particular family, the HD-Zip, is unique to plants. These proteins are characterized by the singular combination of a homeodomain associated to a leucine zipper, which acts as a dimerization domain. Dimerization is a necessary requisite for DNA binding. HD-Zip proteins have been classified into four subfamilies, according to the conservation of the HD-Zip domain, gene structures, additional conserved motifs, and functions. These TFs have been studied by several research groups worldwide contributing to the current knowledge about the structures and functions on these complex regulatory proteins. Here, we summarize the available data on the structural features of homeobox genes and their encoded proteins, attempting to unravel their action mechanisms.