1⟩ Did integral membrane transport proteins arise as an independent protein class or from other types of proteins?
Among the more than 400 families of transport systems in our TC system, very few include integral membrane homologues that function in a capacity other than transport. Moreover, almost all these exceptions are receptors. In some cases, transporters gained receptor functions while retaining their transport functions but in other cases they gained this function while losing the capacity to transport. A loss of transport capacity typically is accompanied by the gain or loss of specific protein domains, in a few cases, another protein with high affinity for the transport protein homologue accounts for the loss of function. Dissociation of such protein-protein complexes or losing the extra domains may restore the lost transport functions.