Complete amino acid sequence of the FK506 and rapamycin binding protein, FKBP, isolated from calf thymus.

FKBP, an 11.8 kD intracellular protein that binds the immunosuppressants FK506 (Kd = 0.4 nM) and rapamycin (Kd = 0.2 nM) with high affinity, was purified to homogeneity from calf thymus. The complete amino acid sequence has been determined by automated Edman degradation of the intact molecule and overlapping fragments generated by proteolytic and chemical cleavage. The analysis revealed a 107 amino acid peptide chain with the following sequence: GVQVETISPGDGRTFPKRGQTCVVHYTGMLEDGKKFDSSRDRNKPFKFV- LGKQEVIRGWEEGVAQMSVGQRAKLTISPDYAYGATGHPGIIPPNATLIFDVELLKLE. The molecular weight, calculated from the amino acid sequence to be 11,778 D, was confirmed by electrospray ionization mass spectrometry. Thus, naturally isolated bovine FKBP does not appear to have any residues modified by glycosylation, phosphorylation, or other posttranslational derivatization processes. Bovine FKBP has only three amino acid residues that differ from human FKBP, whose sequence was elucidated by cloning and sequencing complementary DNA (Standaert et al., 1990). The protein has a substantial number of hydrophilic peptide segments with prevalent beta-strand type of chain fold. Understanding the biological function of FKBP and other members of the immunophilin class and their respective complexes with immunosuppressive drugs may provide insights into cytoplasmic signalling mechanisms, protein folding and translocation, and other cellular processes.