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 proteinprotein 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 handcontaining 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.

A Substrate Preference for the Rough Endoplasmic Reticulum Resident Protein FKBP22 during Collagen Biosynthesis

“…Recombinant human FKBP22 showed a mixed population of monomers and dimers in solution with a shift toward the dimer upon increasing protein concentrations. Strikingly, increased temperature also facilitated the formation of dimers (38).…”

“…The purified protein under non reducing conditions migrates slightly faster than under reducing condition. The two cysteine residues in the sequence form a disulfide bridge (38), resulting in a different migration under reducing and non-reducing conditions. Fig.…”

“…A high content of ␣-helix is observed in FKBP22 compared with the other FKBPs. This is due to the presence of two EF-hand motifs, which have an ␣-helical structure (38). FKBP65 also contains two EF-hand motifs like FKBP22.…”

“…The amino-terminal region of FKBP22 is located on the opposite side of the catalytic site of the FKBP domain, and this region shows major structural differences compared with FKBP12, which did not act as a molecular chaperone in vitro (12). Several side chains of hydrophobic amino acid residue are exposed to the surface (38); therefore, this region may be responsible for the function as a molecular chaperone even in presence of FK506. The influence of the EF-hand motifs on the PPIase activity of FKBP22 was studied in the absence and presence of calcium.…”

“…The influence of the EF-hand motifs on the PPIase activity of FKBP22 was studied in the absence and presence of calcium. The calcium ion is required to coordinate the structure of the EF hands in FKBP22 (38). The ␣-helical structures of the EF-hand motif disappears after removing calcium (Fig.…”