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Tetratricopeptide
The tetratricopeptide repeat (TPR) is a structural motif. It consists in a degenerate 34 amino acid sequence motif identified in a wide variety of proteins. It is found in tandem arrays of 3–16 motifs, which form scaffolds to mediate protein–protein interactions and often the assembly of multiprotein complexes. These alpha-helix pair repeats usually fold together to produce a single, linear solenoid domain called a TPR domain. Proteins with such domains include the anaphase-promoting complex (APC) subunits cdc16, cdc23 and cdc27, the NADPH oxidase subunit p67-phox, hsp90-binding immunophilins, transcription factors, the PKR protein kinase inhibitor, the major receptor for peroxisomal matrix protein import PEX5 and mitochondrial import proteins.
==Structure==
The structure of the PP5 protein was the first structure to be determined. The structure solved by X-ray crystallography by Das and colleagues showed that the TPR sequence motif was composed of a pair of antiparallel alpha helices. The PP5 structure contained 3 tandem TPR repeats which showed the sequential TPR repeats formed an alpha-helical solenoid structure.
A typical TPR structure is characterized by interactions between helices A and B of the first motif and helix A’ of the next TPR. Although the nature of such interactions may vary, the first two helices of the TPR motif typically have a packing angle of ~24 degrees within a single motif. Repeats of more than three TPR motifs generate a right handed superhelix characterized by both concave and a convex faces of which the concave face is usually involved in ligand binding. [ウィキペディア（Wikipedia）』]
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