Domain structure in aggregating proteoglycans from cartilage

Abstract

Cartilage is a specialized connective tissue with a biomechanical function which is particularly suited to compressive load bearing, such as in the articular surface of diarthrodial joints. The properties of the tissue depend upon a highly organized extracellular matrix, which contains a dense network of fine collagen fibres embedded in a high concentration of large aggregating proteoglycans. It is the balance between the high osmotic swelling pressure created by the proteoglycans and the tension in the collagen network that results in the compressive properties characteristic of the tissue [ 1 1. The maintenance of the tissue depends on the continued activity of the chondrocytes that produce the components of the matrix. Proteoglycans are constantly turned over even in mature tissue and they are continuously synthesized and secreted by the chondrocytes [ 11. The large aggregating proteoglycan (Fig. 1 ) contains a high molecular mass protein core (230 kDa), t o which is attached, during post-translational processing (mainly in the Golgi), a large number of chondroitin sulphate and keratan sulphate chains which make up 90% of the total mass of the secreted proteoglycan 121. There are also some N-linked and many Winked oligosaccharides attached to the protein core. Link protein is also synthesized by the chondrocytes along the same pathways as proteoglycan [ 3 ] , but the hyaluronan with which they aggregate does not share this pathway as it is synthesized at the chondrocyte plasma membrane and is therefore only available for aggregation after secretion. The extracellular assembly of aggregates, in which proteoglycans bind to hyaluronate, and are stabilized by link protein, provides a mechanism for immobilizing the proteoglycans in the matrix. Investigation of the protein core structure has shown a single gene product which contains several distinct domains (Fig. 1; Table I ) [2, 41. By rotary-shadowing electron microscopy, these appear as two extended and three globular regions (51. The N-terminal region of the protein core contains two globular domains G1 and G2 separated by a 21 nm extended segment. The major extended region bearing much of the keratan sulphate and all of the chondroitin