Abstract
Osteoadherin is a recently described bone proteoglycan containing keratan sulfate. It promotes integrin (alphav beta3)-mediated cell binding (Wendel, M., Sommarin, Y., and Heinegârd, D. (1998) J. Cell Biol. 141, 839-847). The primary structure of bovine osteoadherin has now been determined by nucleotide sequencing of a cDNA clone from a primary bovine osteoblast expression library. The entire translated primary sequence corresponds to a 49,116-Da protein with a calculated isoelectric point for the mature protein of 5.2. The dominating feature is a central region consisting of 11 B-type, leucine-rich repeats ranging in length from 20 to 30 residues. The full, primary sequence contains four putative sites for tyrosine sulfation, three of which are at the N-terminal end of the molecule. There are six potential sites for N-linked glycosylation present. Osteoadherin shows highest sequence identity, 42%, to bovine keratocan and 37-38% identity to bovine fibromodulin, lumican, and human PRELP. Unique to osteoadherin is the presence of a large and very acidic C-terminal domain. The distribution of cysteine residues resembles that of other leucine-rich repeat proteins except for two centrally located cysteines. Northern blot analysis of RNA samples from various bovine tissues showed a 4.5-kilobase pair message for osteoadherin to be expressed in bone only. Osteoadherin mRNA was detected by in situ hybridization in mature osteoblasts located superficially on trabecular bone.
Highlights
Connective tissues are dominated by an extensive extracellular matrix
Osteoadherin is primarily expressed by mature osteoblasts, as shown in studies of its expression by in situ hybridization
Osteoadherin has been shown to be an additional member of the family of leucine-rich repeat proteins in the extracellular matrix
Summary
Connective tissues are dominated by an extensive extracellular matrix. In bone, the matrix is dominated by mineral in the form of hydroxyapatite. The number of LRR proteins in connective tissues with identified primary sequences has increased This group of extracellular matrix molecules with LRRs includes biglycan [10], decorin, fibromodulin [11], lumican [12], PRELP [13], keratocan [14], chondroadherin [15], proteoglycan-Lb [16], and osteoglycin [17]. Except for chondroadherin and PRELP, they are usually substituted with one or a few glycosaminoglycan chains Several of these LRR proteins have been shown to bind components of the extracellular matrix, e.g. collagen as mentioned above [18] and fibronectin [19], growth factors [20, 21], and cells [22, 23]. Osteoadherin is primarily expressed by mature osteoblasts, as shown in studies of its expression by in situ hybridization
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