In
structural biology, a
protein subunit is a single protein molecule that assembles (or "
coassembles") with other protein molecules to form a
protein complex. Some naturally occurring proteins have a relatively small number of subunits and therefore described as
oligomeric, for example
hemoglobin or
DNA polymerase. Others may consist from a very large number of subunits and therefore described as
multimeric, for example
microtubules and other
cytoskeleton proteins. The subunits of a multimeric protein may be identical,
homologous or totally dissimilar and dedicated to disparate tasks.