Absence of the I-10 Protein Segment Mediates Restricted Dimerization of the Cartilage-specific Fibronectin Isoform

The cartilage-specific (V + C) - fibronectin isoform does not efficiently heterodimerize with other V-region splice variants of fibronectin. To understand better the structural elements that determine this restricted dimerization profile, a series of truncated fibronectin expression constructs with various internal deletions in the V, III-15, or I-10 segments were constructed and co-transfected into COS-7 cells with either the V + C + or the (V + C) - isoform. SDS-PAGE and immunoblot analyses of the resulting conditioned media suggest that the I-10 segment must either be present in both monomeric subunits of fibronectin or absent from both subunits for efficient dimerization to occur. Further studies suggest that the Ι-10 segment specifically, not simply a balanced number of type I repeats at the carboxyl terminus of each monomeric subunit, plays an important role in determining different fibronectin dimerization patterns. Neither I-11 nor Ι-12 could be substituted for segment Ι-10 without significantly reducing the formation of heterodimers. Therefore, absence of segment Ι-10 explains why (V + C) - fibronectin is not found in heterodimeric configurations with other native V-region splice variants in cartilage. The unique dimerization pattern of (V + C) - fibronectin does not prevent matrix formation yet is consistent with this isoform having specialized properties in situ that are important for either the structural organization and biomechanical properties of cartilage or the regulation of a chondrocytic phenotype.