Comparison of protein repeat classifications based on structure and sequence families

Tandem repeats (TR) in proteins are common in nature and have several unique functions. They come in various forms that are frequently difficult to recognize from a sequence. A previously proposed structural classification has been recently implemented in the RepeatsDB database. This defines five main classes, mainly based on repeat unit length, with subclasses representing specific folds. Sequence-based classifications, such as Pfam, provide an alternative classification based on evolutionarily conserved repeat families. Here, we discuss a detailed comparison between the structural classes in RepeatsDB and the corresponding Pfam repeat families and clans. Most instances are found to map one-to-one between structure and sequence. Some notable exceptions such as leucine-rich repeats (LRRs) and alpha-solenoids are discussed.