Extended and globular protein domains in cartilage proteoglycans

Abstract

Electron microscopy after rotary shadowing and negative staining of the large chondroitin sulphate proteoglycan from rat chondrosarcoma, bovine nasal cartilage and pig laryngeal cartilage demonstrated a unique multidomain structure for the protein core. A main characteristic is a pair of globular domains (diameter 6-8 nm), one of which forms the N-terminal hyaluronate-binding region. They are connected by a 25 nm-long rod-like domain of limited flexibility. This segment is continued by a 280 nm-long polypeptide strand containing most chondroitin sulphate chains (average length 40 nm) in a brush-like array and is terminated by a small C-terminal globular domain. The core protein showed a variable extent of degradation, including the loss of the C-terminal globular domain and sections of variable length of the chondroitin sulphate-bearing strand. The high abundance (30-50%) of the C-terminal domain in some extracted proteoglycan preparations indicated that this structure is present in the cartilage matrix rather than being a precursor-specific segment. It may contain the hepatolectin-like segment deduced from cDNA sequences corresponding to the 3'-end of protein core mRNA [Doege, Fernandez, Hassell, Sasaki & Yamada (1986) J. Biol. Chem. 261, 8108-8111; Sai, Tanaka, Kosher & Tanzer (1986) Proc. Natl. Acad. Sci. 83, 5081-5085; Oldberg, Antonsson & Heinegård (1987) Biochem. J. 243, 255-259].

Evidence of a defined spatial arrangement of hyaluronate in the central filament of cartilage proteoglycan aggregates

“…This interaction is stabilized by the binding of a small glycoprotein, the link protein [6,8], with affinity for both hyaluronate [9,10] and the hyaluronate-binding region of the aggrecan monomer, leading to the formation of a very stable ternary complex [11]. This model has been extended by a combination of electron microscopy [12][13][14] and sequence analysis at the protein [15] and cDNA [16][17][18][19][20] level.…”

“…At the N-terminus a pair of globular domains, GI and G2, are located, interspaced by an extended domain El [13]. GI corresponds to the hyaluronate-binding region [14] and contains loop structures that are homologous both to G2 and the link protein [15,17].…”

“…globular domain G2 does not bind to hyaluronate [14,21]. G2 is followed by the extended strand E2, constituting both the keratan sulphate-and chondroitin sulphate-attachment regions [13]. E2 is terminated by the C-terminal globular domain, G3, showing sequence homology to a class of mammalian lectins [16][17][18][19][20].…”

Conserved basic residues in the C-type lectin and short complement repeat domains of the G3 region of proteoglycans

“…If the SCR domain is absent from G3, the resulting G3 CRD model has dimensions of 4.9 nmi 4.7 nmi4.1 nm. This might account for the smaller diameter of 3.5p1 nm for the G3 domain reported in [3].…”

“…G1 and G2 are followed by a large extended intermediate region of 1487 residues containing keratan sulphate and chondroitin sulphate attachment sites. The C-terminal G3 region is present when aggrecan is secreted but is lost as cartilage ages [3][4][5]. G3 has been implicated in the intracellular synthesis and maturation of aggrecan, as its absence causes nanomelia, a lethal genetic mutation in chicken [6][7][8].…”