MOLECULAR CHARACTERIZATION OF A FKBP-TYPE IMMUNOPHILIN FROM HIGHER-PLANTS

Immunophilins are intracellular receptors for the immunosuppressants c yclosporin A, FK506, and rapamycin. In addition to their use in organ transplantation, these natural products have been used to investigate signaling pathways in yeast, plant, and mammalian cells. We have recen tly described the identification of an immunosuppressant-sensitive sig naling pathway in and the purification of several immunophilins from V icia faba plants. We now report the molecular characterization of a 15 kDa FK506- and rapamycin-binding protein from V. faba (VfFKBP15). The amino acid sequence deduced from the cDNA starts with a signal peptid e of 22 hydrophobic amino acids. The core region of VfFKBP15 is most s imilar to yeast and mammalian FKBP13 localized in the endoplasmic reti culum (ER). In addition, VfFKBP15 has a carboxy-terminal sequence that is ended with SSEL, a putative ER retention signal. These findings su ggest that VfFKBP15 is a functional homolog of FKBP13 from other organ isms. Interestingly, two distinct cDNAs corresponding to two isoforms of FKBP15 have been cloned from Arabidopsis and also identified from r ice data base, suggesting that pFKBP15 (plant FKBP15) is encoded by a small gene family in plants. This adds to the diversity of plant FKBP members even with the same subcellular localization and is in contrast with the situation in mammalian and yeast systems in which only one F KBP13, the recombinant VfFKPBP15 protein has rotamase activity that is inhibited by both FK506 and rapamycin with a K-i value of 30 nM and 0 .9 mM, respectively, illustrating that VfFKPB15 is ubiquitously expres sed in various plant tissues including leaves, stems, and roots, consi stent with the ER localization of the protein. Levels of VfFKBP15 mRNA are elevated by heat shock, suggesting a possible role for this FKBP member under stress conditions.