Family- and domain-based classifications are not always straightforward and can overlap, since proteins are sometimes assigned to families by virtue of the domain(s) they contain. An example of this kind of complexity is outlined below.
Protein families and domain composition – an example
Regulator of G-protein signalling (RGS) domains are protein structural units that activate GTPases. They are found in sequences that belong to the RGS protein family, which are multi-functional GTPase-accelerating proteins. All RGS protein family members contain an RGS domain, but while some (such as RGS1) consist of little more than the domain, others (such as RGS3 and RGS6) contain additional domains that confer further functions, such as DEP domains which are involved in membrane targeting.
RGS domains are also found in proteins belonging to other families, such as beta-adrenergic receptor kinases, axins, and some members of the sorting nexin family. The family groupings and domain composition of some of these proteins is summarised in Figure 7 below.
Figure 7 Family groupings and domain composition of some RGS domain-containing proteins.