Proline is unique among amino acids for several reasons. The most prominant is proline has the amino nitrogen cyclized with the side chain terminal carbon which leads to many of the unique properties of proline. This greatly restricts the conformational space available to the peptide in which proline occurs. Other effects are a reduced barrier to cis-trans isomers of the peptide bond and a loss of H bonding capability of the immino nitrogen. These effects give proline a unique role in secondary structure elements in which it occurs. Proline also has other roles in structural proteins, signal transduction and other areas. Proline is a component of collagen, a major structural component of cells and animals. SH3(Src homology region 3) and WW domains of signal transduction pathway proteins recognize proline containing sequences and are used to mediate protein-protein interactions of signal transduction components. Proline specific peptdases have been found that are sensitive to the conformation of proline. Cis-trans isomerization of proline is the rate limiting step in folding of some proteins and many cyclophilins or racemases catalyze the cis-trans racemizaton of proline containing proteins.