ALTERNATIVE SPLICING OF VWFA MODULES GENERATES VARIANTS OF TYPE-VI COLLAGEN ALPHA-3 CHAIN WITH A DISTINCTIVE EXPRESSION PATTERN IN EMBRYONIC CHICKEN TISSUES AND POTENTIALLY DIFFERENT ADHESIVE FUNCTION

Type VI collagen, a ubiquitous extracellular cell adhesion molecule, i s formed by heterotrimeric monomers which associate into dimers and te tramers and assemble into larger oligomers constituting the 100 nm-lon g periodic microfilaments of connective tissues. One distinctive struc tural characteristic of type VI collagen is represented by an alpha 3 chain with a much larger molecular mass compared to the other two chai ns and with an extensive size heterogeneity, exemplified by the separa tion into up to five polypeptides in SDS-PAGE. There is evidence that the alpha 3(VI) mRNA can undergo alternative splicing of three VWFA mo dules at the 5'-end, potentially resulting in the expression of protei n variants. Here we report that alternative splicing of alpha 3(VI) mR NA in chicken embryo did not result in the absolute predominance of a particular alpha 3(VI) form in any tissue; instead, the expression of variants including exons A9, A8 and A6 increased with age. In addition , these variants had a more restricted tissue distribution pattern com pared to variants including only constitutive exons: A9+ were the rare st and were present almost exclusively in skin and skeletal muscle; A6 + were expressed in several of the examined tissues with local variati ons; A8+ had intermediate levels and were less widely distributed than A6+ variants. Quantitative densitometric scanning of immunoblots of t ype VI collagen purified from gizzard and stained with VWFA module-spe cific antibodies indicated that the polymorphic migration pattern of a lpha 3(VI) polypeptides is contributed by concurrent or independent sp licing of two exons (A8 and A6) and probably by processing and/or prot eolysis at the N- and C-terminus. Three exon-specific recombinant poly peptides were examined in cell adhesion assays, and A6 appeared to be the most active, particularly at low substrate concentrations. The adh esion to the recombinant modules was not abrogated by EDTA nor by mAbs against the integrin beta 1 or alpha 2 subunits. Over all, these resu lts suggest that the splicing of the alpha 3(VI) mRNA and the tissue d istribution pattern of type VI collagen variants, apart from promoting cell adhesion to different extents, might also affect additional stru ctural as well as functional properties of this molecule, including mi crofilament formation and interaction with other extracellular matrix molecules.