Diagram of a type IX collagen molecule located on the surface of a cartilage fibril containing quarter-staggered type II collagen molecules. The pepsin-resistant fragments LMW and HMW are shown above the diagram. LMW represents the disulfide-bonded COL1 domain of the intact type IX molecule. HMW represents the COL2 and COL3 domains. Note that pepsin removes a portion of the NC3 domain of the¢ 2 (IX) chain while leaving the NC3 domains of the (~ l (IX) and (x3 (IX) chains undigested. cDNA clones led to the complete characterization of two of the three genetically distinct polypeptide subunits of type IX collagen, and the identity of type IX collagen with the proteoglycan PG-Lt was established". Analysis of clones containing the genes encoding aI (IX) and a2 (IX) further revealed that the number and size of the exons of these genes are distinct from those in fibrillar collagen genes, indicating that the type IX collagen genes belong to a (a)

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