common structural motif is a transmembrane protein

An especially common structural motif
is a transmembrane protein containing seven !
helices within the plasma membrane. The
amino end is extracellular; the carboxy end is
intracellular. Different oligosaccharide side
chains are usually bound to the extracellular
domains. The intracellular domains have binding
sites for other molecules involved in signal
transmission. The seven-helix motif is the
characteristic structure of G protein-binding receptors
(p. 268). As the G proteins themselves,
these receptors and their genes form a large
family with a long evolutionary history. In
yeast, they serve to discern the pheromones of
the mating types (p. 186); in higher organisms
they are the basis for transmitting signals of vision,
smell, and taste