Structural analysis of glycosylation inhibiting factor

We have previously cloned a cDNA encoding glycosylation inhibiting factor (GIF), which appears to be a subunit of antigen-specific suppressor T cell factors (TsFs). We succeeded in crystallization of the recombinant human GIF (recGIF) expressed in E. coli. High resolution crystallographic analysis revealed that 13 kDa recGIF peptides noncovalently interact and form a trimer of barrel shape. Three p-sneets, each of which consists of four strands in the same monomer, and two strands from the other two subunits, make up a barrel, and six ex-helices surround the outside. The overall topology is quite unique because this barrel is constructed from three independent non parallel β-sheets. The overall structure of this trimeric GIF molecule resembles other trefoil cytokines such as interleukin-lβ (-IL-Iβ) and fibroblast growth factor (FGF). The fact that these proteins lack signal peptide suggest that the three-fold symmetry structure relate to secretion via non classical pathway without secretory leader sequence. The β-a-β motifs of the GIF molecule are resemble both interleukin-8 (IL-8) and the peptide binding domain of the major histocompatibility complex (MHC). Light scattering analysis indicated that recGIF forms tnmer in solution. There was no inter- or intramolecular disulfide bond in GIF molecule. Among the three cysteine residues, only CysoO, whose sulfhydryl group locates between the two a-helices, is highly susceptible to S-alkylation, suggesting that the two a-helices move flexibly. Accumulating evidence suggested that posttranslational modification via secretion is required for full bioactivity of GIF protein. Indeed, the molecular weight of Ts-hybridoma derived GIF molecule is slightly higher than the theoretical mass. The fine structural analyses of the Ts-hybndoma derived GIF are in progress.