Plant immunophilins: functional versatility beyond protein maturation

Summary 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. Contents Summary 753 I. A historical perspective 754 II. The mechanism of immunosuppression 754 III. PPIase activity and protein folding 755 IV. Immunophilins are ubiquitous proteins 755 V. Plant immunophilins 757 VI. Specific functions of plant immunophilins 759 VII. Concluding remarks 765 References 765