Covalent Cross-linking of the NC1 Domain of Collagen Type IX to Collagen Type II in Cartilage

David R Eyre,Terri Pietka,Mary Ann Weis,Jiann-Jiu Wu

doi:10.1074/jbc.m311653200

Abstract

From a study to understand the mechanism of covalent interaction between collagen types II and IX, we present experimental evidence for a previously unrecognized molecular site of cross-linking. The location relative to previously defined cross-linking sites predicts a specific manner of interaction and folding of collagen IX on the surface of nascent collagen II fibrils. The initial evidence came from Western blot analysis of type IX collagen extracted by pepsin from fetal human cartilage, which showed a molecular species that had properties indicating an adduct between the alpha1(II) chain and the C-terminal domain (COL1) of type IX collagen. A similar component was isolated from bovine cartilage in sufficient quantity to confirm this identity by N-terminal sequence analysis. Using an antibody that recognized the putative cross-linking sequence at the C terminus of the alpha1(IX) chain, cross-linked peptides were isolated by immunoaffinity chromatography from proteolytic digests of human cartilage collagen. They were characterized by immunochemistry, N-terminal sequence analysis, and mass spectrometry. The results establish a link between a lysine near the C terminus (in the NC1 domain) of alpha1(IX) and the known cross-linking lysine at residue 930 of the alpha1(II) triple helix. This cross-link is speculated to form early in the process of interaction between collagen IX molecules and collagen II polymers. A model of molecular folding and further cross-linking is predicted that can spatially accommodate the formation of all six known cross-linking interactions to the collagen IX molecule on a fibril surface. Of particular biological significance, this model can accommodate potential interfibrillar as well as intrafibrillar links between the collagen IX molecules themselves, so providing a mechanism whereby collagen IX could stabilize a collagen fibril network.

Highlights

Collagen type IX is a member of the fibril-associated collagen with interrupted triple helix (FACIT)1 family of collagen molecules, which share homologous domains and are all believed to function in the extracellular matrix in association
Analysis of the structures of cross-linked peptides isolated from type IX collagen showed one class that had derived from the reaction of a hydroxylysine aldehyde in the N-telopeptide of type II collagen to a hydroxylysine at the N terminus of the triple helical COL2 domain (4 – 6)
MAb 2B4, which was prepared against the type II collagen C-telopeptide domain, recognizes the cleaved C terminus of the sequence EKGPDP [17]

Summary

Introduction

Collagen type IX is a member of the fibril-associated collagen with interrupted triple helix (FACIT)1 family of collagen molecules, which share homologous domains and are all believed to function in the extracellular matrix in association with collagen fibril surfaces [1, 2]. The initial evidence came from Western blot analysis of type IX collagen extracted by pepsin from fetal human cartilage, which showed a molecular species that had properties indicating an adduct between the ␣1(II) chain and the C-terminal domain (COL1) of type IX collagen.

Results

Conclusion