Deletion of the C-terminal 138 amino acids of the wheat FKBP73 abrogates calmodulin binding, dimerization and male fertility in transgenic rice

Wheat FKBP73 (wFKBP73) belongs to the FK506-binding protein (FKBP) family which, in common with the cyclophilin and parvulin families, possesses peptidyl prolyl cis-trans isomerase (PPIase) activity. Wheat FKBP73 has been shown to contain three FKBP12-like domains, a tetratricopeptide repeat (TPR) via which it binds heat shock protein 90 and a calmodulin-binding domain (CaMbd). In this study we investigated: (1) the contribution of the N-terminal and C-terminal moieties of wFKBP73 to its biological activity by over-expression of the prolyl isomerase domains in transgenic rice, and (2) the biochemical characteristics of the C-terminal moiety. The recombinant wFKBP73 was found to bind calmodulin via the CaMbd and to be present mainly as a dimer in solution. The dimerization was abrogated when 138 amino acids from the C-terminal half were deleted. Expression of the full-length FKBP73 produced fertile rice plants, whereas the expression of the peptidyl prolyl cis-trans isomerase domains in transgenic rice resulted in male-sterile plants. The male sterility was expressed at various stages of anther development with arrest of normal pollen development occurring after separation of the microspores from the tetrads. Although the direct cause of the dominant male sterility is not yet defined, we suggest that it is associated with a novel interaction of the prolyl isomerase domains with anther specific target proteins.