This is the highest level of
protein structure. The quaternary structure describes how proteins assemble together. A protein which stands alone is referred to as a
monomer. Some proteins
polymerize into long chains such as
actin which is involved in some way in just about every
cell process. Some proteins just assemble in pairs
(dimers) or in groups of three
(trimers). If two different proteins pair up, its a
heterodimer, the same two - a
homodimer, and I'm sure you can imagine the rest.
While the tertiary structure is often sufficient to explain the function of monomeric (no pairing) proteins, many proteins do assemble into higher order clusters which then modulate their function.
See also:
primary structure
secondary structure
tertiary structure