Leucine zipper dimer

Most DNA-binding regulatory proteins recognize
specific sites as dimers. One part of the
molecule serves as the recognition molecule,
the other stabilizes the structure. A particularly
striking example is given by proteins with a
leucine zipper motif. The name is derived from
the basic structure. Two ! helices are joined like
a zipper by periodically repeated leucine residues
located at the interface of the two helices.
The two helices separate, form a Y-shaped
structure, and extend into the major groove of
the DNA (1). Leucine zipper proteins may be homodimers
with identical subunits (2, 3) or heterodimers
with different albeit similar subunits
(4). The ability to form unlike dimers (heterodimerization)
greatly expands the spectrum of
specificites. The use of combinations of different
proteins to control cellular functions is
called combinatorial control. (Figure redrawn
from Alberts et al., 1994).
A DNA-binding motif related to the leucine zipper
is the helix–loop–helix (HLH) motif (not
shown). The HLH motif consists of one short !
helix and one longer ! helix. The two ! helices
are connected by a flexible loop of protein.