SAXS Models of TGFBIp Reveal a Trimeric Structure and Show That the Overall Shape Is Not Affected by the Arg124His Mutation

Human transforming growth factor β induced protein (TGFBIp) is composed of 683 residues, including an N-terminal cysteine-rich (EMI) domain, four homologous fasciclin domains, and an Arg-Gly-Asp (RGD) motif near the C-terminus. The protein is of interest because mutations in the TGFBI gene encoding TGFBIp lead to corneal dystrophy (CD), a condition where protein aggregates within the cornea compromise transparency. The complete three-dimensional structure of TGFBIp is not yet available, with the exception of a partial X-ray structure of the archetype FAS1 domain derived from Drosophila fasciclin-1. In this study, small-angle X-ray scattering (SAXS) models of intact wild-type (WT) human TGFBIp and a mutant (R124H) are presented. The mutation R124H leads to a variant of granular CD. The deduced structure of the TGFBIp monomer consists of four FAS1 domains in a simple “beads-on-a-string” arrangement, constructed by the superimposition of four consecutive Drosophila fasciclin domains. The SAXS-based model of the TGFBIp R124H mutant displayed no structural differences from WT. Both WT TGFBIp and the R124H mutant formed trimers at higher protein concentrations. The similar association properties and three-dimensional shape of the two proteins suggest that the mutation does not induce any major structural rearrangements, but points towards the role of other corneal-specific factors in the formation of corneal R124H deposits.

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► Mutations in the corneal protein TGFBIp lead to CDs, in which progressive accumulation of TGFBIp aggregates eventually leads to blindness.
► No cure currently exists for CDs, except for corneal transplant. The mechanism by which point mutations cause pathology is unknown.
► Here we use SAXS to provide the first low-resolution structure of TGFBIp, which reveals a beads-on-a-string arrangement of the protein's four domains.
► Monomers associate, likely in an end-to-end arrangement, to form trimers over time.
► The disease-linked mutant R124H shows the same structure, indicating that the R124H deposits in the cornea are not caused by major structural rearrangements.