Leucine Zippers

Abstract

Abstract The leucine zipper (ZIP) motif consists of a periodic repetition of a leucine residue at every seventh position (heptad repeat) and forms an α‐helical conformation, which facilitates dimerisation and in some cases higher oligomerisation of proteins by forming a parallel helix–helix association stabilised by formation of an interhelical hydrophobic core involving leucine side chains. These nonpolar interactions are important for stabilisation of the ZIP structures, whereas specificity of these dimerisation and oligomerisation is mediated by electrostatic interactions, such as formation of salt bridges. In many eukaryotic gene regulatory proteins, the ZIP motif is flanked at its N ‐terminus by a basic region containing characteristic residues that facilitate deoxyribonucleic acid (DNA) binding and is referred to as bZIP. ZIP‐related heptad repeats are frequently found in a variety of proteins and catalyse dimerisation/oligomerisation by forming both parallel and antiparallel coiled coils. Key Concepts: A heptad repeat of leucine residues, leucine zipper (ZIP), is an important sequence motif facilitating protein–protein interactions. ZIP forms an amphiphilic α helical structure, in which two residues that are separated by seven residues in sequence are located at nearly the same molecular surface in an α helix. The amphiphilic nature of the ZIP helix facilitates protein dimerisation by forming a parallel helix–helix association. Leucine side chains extending from one α helix interdigitate with those displayed from an α helix of a second polypeptide like a zipper to stabilise formation of an interhelical hydrophobic core. The specificity of the ZIP dimerisation is mediated by electrostatic interactions, such as formation of salt bridges. ZIP is frequently fused with other sequence motifs to produce biologically important functional structural modules. The ‘basic region ZIP’ (bZIP motif) is a fusion between a segment that is rich in basic residues facilitating DNA binding and ZIP, forming a class of eukaryotic transcription factors. The DNA binding specificity of bZIP is determined by the basic region that directly binds a class of palindromic DNA sequences. ZIP‐related sequence motifs containing an incomplete heptad repeat are frequently found in a variety of proteins and catalyse dimerisation/oligomerisation by forming both parallel and antiparallel coiled coils.