Purpose. A novel cornea-associated antigen (CO-Ag) has been purified from stromal extracts. The protein is the target for autoantibodies in patients with Mooren's ulcer. In the present study the amino acid sequence of CO-Ag was determined, the structure-function properties of CO-Ag was elucidated, and a search for sequence homology with the hepatitis C viral (HCV) polyprotein was performed. Methods. Purified CO-Ag was subjected to N-terminal sequencing by automated Edman degradation. Binding of calcium (Ca2+) to CO-Ag was measured by a direct 45Ca2+-binding assay. A directed computer-based homology analysis of CO-Ag and the HCV polyprotein was performed to detect possible substrates for molecular mimicry. Results. The complete amino acid sequence of CO-Ag reveals that it is closely related to the Ca2+-binding protein of the S-100 family. The sequence of CO-Ag shows a high homology with calgranulin C (CaG-C) previously purified from pig granulocytes. The functional Ca2+-binding sites of CO-Ag and CaG-C were different based on homology with known Ca2+-binding domains and their Ca2+-binding properties. The number of Ca2+-binding sites for the CO-Ag subunit is about half of the CaG-C monomer, although these two proteins have a similar binding constant of 2 × 10-4 M. Five consecutive amino acid changes from D63-K-K-G-A67 in CO-Ag to M63-Q-D-E-Q67 occurred in CaG-C in the C-terminal Ca2+-binding domain. These differences alter the structure of CO-Ag which no longer can bind Ca2+ ions. More importantly, this altered region of CO-Ag is shared by the NS-3 protein of HCV. NS-3 is one of the most potent immunogenic proteins for T cells and B cells observed in patients infected with HCV. However, this region of similarity is located only at the HCV-J6 sequence (residues 1149-1153). Conclusions. The structure of CO-Ag is chimeric in nature: the N-terminal Ca2+-binding domain strongly resembles CaG-C and other S-100 proteins which are believed to function as Ca2+-mediators. The C-terminal domain contains a non-functional Ca2+-binding site and is the likely area of the immunogenic and/or the cross-reacting epitopes responsible for Mooren's ulcer.
|Original language||English (US)|
|Journal||Investigative Ophthalmology and Visual Science|
|State||Published - Feb 15 1996|
ASJC Scopus subject areas
- Sensory Systems
- Cellular and Molecular Neuroscience