The carboxyl terminus of the chicken alpha 3 chain of collagen VI is a unique mosaic structure with glycoprotein Ib-like, fibronectin type III, and Kunitz modules.

The primary amino acid sequence of the carboxyl-terminal portion of the alpha 3 chain of chick type VI collagen as deduced from the nucleotide sequence is reported. This carboxyl-terminal segment is not present in the alpha 1 and alpha 2 chains of chick type VI collagen and is specific for a mosaic region with extensive similarities to several other proteins. This unique segment, beginning with a stretch (73 residues) very rich in serine and threonine, is preceded by sequences analogous to the platelet glycoprotein Ib. This region is followed by one segment that closely resembles the type III domains of fibronectin. At the end of the sequence, there is a 58-residue motif very similar to sequences characteristic of the Kunitz-type proteinase inhibitors. The present findings and our recent observation that the alpha 3(VI) chain contains 11 repeats similar to type A repeats of von Willebrand factor raise interesting questions about the peculiar mosaic structure and the multiple functions that this unique collagen might play in growth and remodeling of connective tissues.