The Elastin-like Protein Matrix of Lamprey Branchial Cartilage Is Cross-Linked by Lysyl Pyridinoline

Abstract

The cranial skeleton of the lamprey, a primitive vertebrate, consists of cartilaginous structures that differ from vertebrate cartilages in having a noncollagenous extracellular matrix. Novel matrix proteins found in these cartilages include lamprin in the annular cartilage and an unidentified protein in the branchial cartilages. Both show biochemical similarities to elastin. The inextractability of these proteins, even to chemical cleavage by cyanogen bromide, indicates a polymer with extensive covalent cross-linking. Here we report on the type of cross-linking. Lysyl pyridinoline was found in high concentration in the elastin-like protein of lamprey branchial cartilage at a ratio of 7:1 to hydroxylysyl pyridinoline, the form that dominates in vertebrate collagens. Both forms of pyridinoline cross-link were absent from annular cartilage and desmosine cross-links, which are characteristic of vertebrate elastin, were not detected in either form of lamprey cartilage. Pyridinoline cross-links are considered to be characteristic of collagen, so their presence in an elastin-like protein in a primitive cartilage poses evolutionary questions about the tissue, the protein, and the cross-linking mechanism.