Molecular properties of wild-type and mutant βIG-H3 proteins

PURPOSE. βIG-H3 is a TGF-β-induced cell adhesion molecule, the mutations of which are responsible for a group of 5q31-linked corneal dystrophies. The characteristic findings in these diseases are accumulation of protein deposits of different ultrastructures. To understand the mechanisms of protein deposits in 5q31-linked corneal dystrophies, the molecular properties of βIG-H3 and the effects of mutation on these properties were studied in vitro. METHODS. Substitution mutations were generated by two-step PCR. Wild-type and mutant recombinant βIG-H3 proteins were raised in Escherichia coli. For structural study, nondenaturing gel electrophoresis, cross-linking experiments, and electron microscopy examination were performed. A solid-phase interaction assay was performed for the interaction of βIG-H3 with other matrix proteins. Wild-type and mutant βIG-H3 cDNAs were cloned into a mammalian expression vector and overexpressed in the corneal epithelial cells by transient transfection. Immunoprecipitation and immunoblot analysis were performed with an antibody against human βIG-H3. Cell adhesion was assayed by measuring enzyme activities of N-acetyl-β-D-glucosaminidase. RESULTS. The recombinant βIG-H3 protein self-assembled to form multimeric bands and appeared to have a fibrillar structure. Solid-phase in vitro interaction assay showed that it bound strongly to type I collagen, fibronectin, and laminin; moderately to collagen type II and VI; and minimally to collagen type IV. Five recombinant mutant forms of βIG-H3 (R124C, R124H, R124L, R555W, and R555Q) commonly found in 5q31-linked corneal dystrophies did not significantly affect the fibrillar structure, interactions with other extracellular matrix proteins, or adhesion activity in cultured corneal epithelial cells. In addition, the mutations apparently produced degradation products similar to those of wild-type βIG-H3. CONCLUSIONS. βIG-H3 polymerizes to form a fibrillar structure and strongly interacts with type I collagen, laminin, and fibronectin. Mutations found in the 5q31-linked corneal dystrophies do not significantly affect these properties. The results suggest that mutant forms of βIG-H3 may require other corneaspecific factors, to form the abnormal accumulations in 5q31-linked corneal dystrophies.