WW domain

The WW domain (also known as WWP domain) is a protein domain with two highly conserved tryptophans that binds proline-rich peptide motifs. The WW domain binds to proteins with particular proline-motifs, [AP]-P-P-[AP]-Y, and/or phosphoserine- phosphothreonine-containing motifs.

Structure
The WW domain is a short conserved region in a number of unrelated proteins, which folds as a stable, triple stranded beta sheet. This short domain of approximately 40 amino acids, may be repeated up to four times in some proteins. The name WW or WWP derives from the presence of two signature tryptophan residues that are spaced 20-23 amino acids apart and are present in most WW domains known to date, as well as that of a conserved proline. The first tryptophan binds to the C-terminal conserved proline and forms part of the domains small hydrophobic core. It is frequently associated with other domains typical for proteins in signal transduction processes.

Examples
A large variety of proteins containing the WW domain are known. These include; dystrophin, a multidomain cytoskeletal protein; utrophin, a dystrophin-like protein of unknown function; vertebrate YAP protein, substrate of an unknown serine kinase; Mus musculus (Mouse) NEDD4, involved in the embryonic development and differentiation of the central nervous system; Saccharomyces cerevisiae (Baker's yeast) RSP5, similar to NEDD4 in its molecular organization; Rattus norvegicus (Rat) FE65, a transcription-factor activator expressed preferentially in liver; Nicotiana tabacum (Common tobacco) DB10 protein, amongst others.