Originally identified as the cellular targets of immunosuppressant drugs, the 
immunophilins encompass two ubiquitous protein families: the FK-506 binding 
proteins or FKBPs, and the cyclosporin-binding proteins or cyclophilins. Present 
in organisms ranging from bacteria to animals and plants, these proteins are 
characterized by their enzymatic activity; the peptidyl-prolyl cis-trans 
isomerization of polypeptides. Whilst this function is important for protein 
folding, it has formed the functional basis for more complex interactions 
between immunophilins and their target proteins. Beginning with a brief 
historical overview of the immunophilin family, and a representative 
illustration of the current state of knowledge that has accumulated for these 
proteins in diverse organisms, a detailed description is presented of the recent 
advances in the elucidation of the role of this ubiquitous protein family in 
plant biology. Though still in its infancy, investigation into the function of 
plant immunophilins has so far yielded interesting results--as a significant 
component of the chloroplast proteome, the abundance of immunophilins located in 
the thylakoid lumen suggests that these proteins may play important roles in 
this relatively uncharacterized subcellular compartment. Moreover, the 
importance of the complex multidomain immunophilins in functions pertaining to 
development is underscored by the strong phenotypes displayed by their 
corresponding mutants.