Structure of human peptidyl-prolyl cis-trans isomerase FKBP22 containing two EF-hand motifs

Abstract

The FK506-binding protein (FKBP) family consists of proteins with a variety of protein-protein interaction domains and versatile cellular functions. It is assumed that all members are peptidyl-prolyl cis-trans isomerases with the enzymatic function attributed to the FKBP domain. Six members of this family localize to the mammalian endoplasmic reticulum (ER). Four of them, FKBP22 (encoded by the FKBP14 gene), FKBP23 (FKBP7), FKBP60 (FKBP9), and FKBP65 (FKBP10), are unique among all FKBPs as they contain the EF-hand motifs. Little is known about the biological roles of these proteins, but emerging genetics studies are attracting great interest to the ER resident FKBPs, as mutations in genes encoding FKBP10 and FKBP14 were shown to cause a variety of matrix disorders. Although the structural organization of the FKBP-type domain as well as of the EF-hand motif has been known for a while, it is difficult to conclude how these structures are combined and how it affects the protein functionality. We have determined a unique 1.9 Å resolution crystal structure for human FKBP22, which can serve as a prototype for other EF hand-containing FKBPs. The EF-hand motifs of two FKBP22 molecules form a dimeric complex with an elongated and predominantly hydrophobic cavity that can potentially be occupied by an aliphatic ligand. The FKBP-type domains are separated by a cleft and their putative active sites can catalyze isomerazation of two bonds within a polypeptide chain in extended conformation. These structural results are of prime interest for understanding biological functions of ER resident FKBPs containing EF-hand motifs.

Keywords: EF-hand motif; Ehlers-Danlos syndrome; FKBP14; FKBP22; crystal structure; peptidyl-prolyl cis-trans isomerase.

Figure 1

FKBP22 crystal structure. (A) Stereo cartoon topology diagram of FKBP22 A chain (white) and FKBP22 B chain (orange) superimposed using residues 7–116 of the FKBP domain. Calcium ions are shown as spheres and labeled 1, 2, and 3. Disulfide bond C19–C77 is shown in cyan. N- and C-termini are labeled as N and C. (B) Stereo surface diagram of FKBP22 dimer composed of chain A (white) and chain B (orange). The surface of potential PPIases active site of the FKBP domain is shown in magenta. An interactive view is available in the electronic version of the article.

Figure 2

(A) The third calcium ion (magenta) in chain A. (B) The same region in chain B is occupied by two water molecules. (C) Electron density for PEG and 1,2-propanediol molecules calculated using 2Fo-Fc coefficients contoured at 1σ and shown in black on the surface of the EF-hand domain of chain A, which is colored based on electrostatic potential value. (D) The same as in (C) shown for chain B. An interactive view is available in the electronic version of the article.