Friends and relations of the cystatin superfamily—new members and their evolution

The cystatin “superfamily” encompasses proteins that contain multiple cystatin‐like sequences. Some of the members are active cysteine protease inhibitors, while others have lost or perhaps never aquired this inhibitory activity. In recent years, several new members of the superfamily have been characterized, including proteins from insects and plants. Based on partial amino acid homology, new members, such as the invariant chain (Ii), and the transforming growth factor‐β receptor type II (TGF‐β receptor II) may, in fact, represent members of an emerging family within the superfamily that may have used some common building blocks to form functionally diverse proteins. Cystatin super‐family members have been found throughout evolution and members of each family of the superfamily are present in mammals today. In this review, the new and older, established members of the family are arranged into a possible evolutionary order, based on sequence homology and functional similarities.