leucine zipper

Picture two long zipper teeth that interlock not with metal but with a row of greasy knobs, and once they lock together the pair stands up like a wishbone and reaches down to clasp something below. That is the leucine zipper: a way for two protein chains to clip onto each other and, joined, to grab DNA in a pincer grip.

Each chain is a single long alpha-helix. Running down one face of that helix, every seventh amino acid is a leucine — a small oily side chain — so the leucines line up in a stripe. When two such helices meet, their leucine stripes mesh and interlock like the teeth of a zipper, holding the two chains together as a coiled-coil. That handshake is the dimerization part; it does not touch DNA at all. Just past it, each chain continues into a stretch rich in basic, positively charged amino acids, and these two basic regions splay apart like the open arms of a Y and lie into the DNA grooves on either side, reading the sequence. Because the whole DNA-binding piece only works as a pair, the proper name is the basic leucine zipper, or bZIP, and the binding site is usually a short, near-symmetric sequence that suits the two-armed grip.

The clever consequence is combinatorial. Because the zipper joins two chains, and many bZIP proteins can pair both with their own kind (a homodimer) and with different partners (a heterodimer), a modest set of bZIP proteins can mix into a much larger set of distinct dimers, each preferring a slightly different DNA site and each switching a different program. The Fos and Jun proteins of the AP-1 factor are the textbook case: Jun can pair with Jun, but Fos-Jun pairs are far more potent, so which partners are around tunes the cell's response to growth signals. It is a small motif with outsized reach into how cells decide to grow and divide.