Two different types of leucine zippers are associated with the homeodomain (HD) in START proteins from plants. (a) Alignment of a region from 16 Arabidopsis proteins illustrating the carboxy-terminal end of the HD adjacent to a ZLZ motif. The leucine zipper region contains three repeats, separated by a loop of around 10-20 amino acids, and followed by another three repeats. Consistent with the hypothesis of α helix formation, no helix-disrupting proline or glycine residues are present in these heptad repeats. The loop region is partially conserved and contains a pair of invariant cysteine residues (CXXC) (gray shading) with a propensity for disulfide linkage predicted to stabilize the structure. (b) Alignment of the basic region leucine zipper (BRLZ) (SMART) and basic-leucine zipper (bZIP) (Pfam), against a similar region in five Arabidopsis proteins. The leucine zipper region contains five repeats preceded by a basic region and the tail end of the HD. The leucines (yellow) and 'a' and 'd' positions of the leucine zippers are marked in both alignments.
Schrick et al. Genome Biology 2004 5:R41 doi:10.1186/gb-2004-5-6-r41