Uncoupling proteins (UCPs) belong to a distinct cluster of the mitochondria
l anion carrier family. Up to five different uncoupling protein types were
found in mitochondria of mammals and plants, and recently in fishes, fungi
and protozoa. They exhibit a significantly conserved structure with several
motifs specific to either the whole cluster or protein type. Uncoupling pr
oteins, as well as the whole mitochondrial anion carrier gene family, proba
bly emerged in evolution before the separation of animal, fungi, and plant
kingdoms and originate from an anion/nucleotide or anion/anion transporter
ancestor. Mammalian UCP1. UCP2, UCP3, and plant uncoupling proteins pUCP1 a
nd pUCP2 are similar and seem to form one subgroup, whereas UCP4 and BMCP1
belong to a different group. Molecular, biochemical, and phylogenic data su
ggest that UCP2 could be considered as an UCP-prototype. UCP1 plays its bio
logical role mainly in the non-shivering thermogenesis while the role of th
e other types is unknown. However, hypotheses have suggested that they are
involved in the general balance of basic energy expenditure, protection fro
m reactive oxygen species, and, in plants, in fruit ripening and seed ontog
eny.