Structural characterization of two variants of fibulin-1 that differ in nidogen affinity

Two C-terminal variants C and D of mouse fibulin-1 were purified from the culture medium of stably transfected human kidney cell clones. They showed, after rotary shadowing, a dumbbell-like structure of about 33 nm in length. Pepsin digestion demonstrated stability of the disulfide-bonded domains 1 (anaphylatoxin-like) and II (multiple EGF-like motifs) but not for domain III which is different in the variants. A close similarity of the variants was observed in immunochemical assays indicating that domain III epitopes are not very antigenic. Binding analysis in solid phase assays demonstrated for variant C a 100-fold stronger binding to the basement membrane protein nidogen than for variant D. Both interactions were sensitive to EDTA. Surface plasmon resonance assays confirmed this difference and showed KD = 60 nM for variant C and KD > 1 microM for variant D. Lower binding activities and smaller differences between both variants were observed for the calcium-dependent binding to fibronectin, laminin-1 and collagen IV. Self aggregation into nest-like oligomers was observed at high concentrations of fibulin-1 which was not sensitive to EDTA.