Identification and characterization of asporin - A novel member of the leucine-rich repeat protein family closely related to decorin and biglycan

Asporin, a novel member of the leucine-rich repeat family of proteins, was partially purified from human articular cartilage and meniscus. Cloning of human and mouse asporin cDNAs revealed that the protein is closely related to decorin and biglycan. It contains a putative propeptide, 4 amino-termina l cysteines, 10 leucine-rich repeats, and 2 C-terminal cysteines. In contra st to decorin and biglycan, asporin is not a proteoglycan. Instead, asporin contains a unique stretch of aspartic acid residues in its amino-terminal region. A polymorphism was identified in that the number of consecutive asp artate residues varied from 11 to 15. The 8 exons of the human asporin gene span 26 kilobases on chromosome 9q31.1-32, and the putative promoter regio n lacks TATA consensus sequences. The asporin mRNA is expressed in a variet y of human tissues with higher levels in osteoarthritic articular cartilage , aorta, uterus, heart, and liver. The deduced amino acid sequence of aspor in was confirmed by mass spectrometry of the isolated protein resulting in 84% sequence coverage. The protein contains an N-glycosylation site at Asn( 281) with a heterogeneous oligosaccharide structure and a potential O-glyco sylation site at Ser(54). The name asporin reflects the aspartate-rich amin o terminus and the overall similarity to decorin.